Sam doesn't resent Mary and Cas for not being able to deal with Nick. He doesn't resent them for bringing up that they can't handle him, either, because Sam wants them to talk about it to him, wants to find a way to make himself stronger so they don't have to tiptoe around the subject, and he knows why they have insisted on telling him so directly- they want Sam to stop being the one to handle him, for his own safety.
But Sam rebuffs them, certain and unyielding and they stop when they realize it's not helpful, just exhausting.
They don't want him tending to him for obvious reasons, but this is Sam's choice, and he needs to help him, even though every minute he spends with him dredges a lot back up. But it helps. Sam has to believe it helps both himself and Nick. Talking to him. Working through their shared experiences. Disconnecting the human from the Archangel who stole his body and claimed it just like he claimed Sam's.
Mary can't look at Nick because she sees a parent as weak as herself, because she sees another hapless soul who got tricked into letting Lucifer wear them, just like Lucifer wore her once, and because of guilt. If not for her, Crowley, and Bobby trying to weaken Lucifer in a last ditch effort by putting Nick's soul back inside his remade body to weaken him, Nick would've stayed dead, as safe as every other soul out there, not that any of them were safe, with Lucifer running Heaven and Hell and able to use them as he pleased.
So she won't look at him, not at all. She feels this is her fault, for Nick going under, for being a sacrifice just so her second-born son wouldn't be, anymore.
Sam wouldn't have okayed that plan if he knew about it, but he doesn't want Mary near Nick for the same reason Nick doesn't want to be near her.
And Sam knows why Cas can't deal with him, even though he will, if Sam is unable to some days.
Every time Cas looks at Nick, he sees himself. Sees how he let Lucifer back out from the Cage by saying yes, and how everything happened after, how everything they've been through, was a direct result of that yes, even if Lucifer probably would've found another way to get out. God probably would've sprung him from the Cage on his own to have Lucifer fight the Darkness eventually- Sam believes that, after everything. But Cas still bears the weight of what was done to Sam, in his body, and guilt, and feelings of failure. It's easier to blame Nick, just as he blames himself, or like how Dean blamed himself for letting Michael in back in Apocalypse world.
Truthfully, Sam thinks they're a bit too similar in that way, and that's half the reason they can't interact.
Nick and Sam are different that way, too.
Sam knows he isn't responsible for whatever Lucifer decided to use himself or anyone else for. Lucifer was persuasive, at least until he had what he wanted, and didn't need to be persuasive anymore, even if he kept up his attempts with Sam to try and wear him down, to make him give himself over like Lucifer wanted. Lucifer wanted love and obedience and control, always. And he knew how to get under people's skin and make people see things his way. That's why God had to lock him up in the first place, even if Sam is sure now that if he didn't, nothing would have been the same. But God was not one for doubt, and he knew his son was too convincing, and even when he wasn't convincing, he was too powerful, too old, and could brute force his way to victory.
It's easier, for Nick, to pretend he had some control, and that Lucifer couldn't do whatever he wanted.
But Nick needs to learn to accept the truth.
Sam's just not sure how he can get him to see it, and he doesn't want to push too hard.
Jessica offers to help, but more often than not, they all agree it's a bad idea for her and Nick to be in the same room. They tend to set each other off in closed spaces, or at the wrong time.
Sam can't deal with both of them at the same time sometimes. It makes him doubt reality when they are in the same room, and the last thing he needs is to mentally flashback to the Cage and for his mind to project Lucifer wearing one of them so they all hear and see the hallucination mocking them. That just sets them all off.
The other issue is Sam is still so much older than them. He's lived thousands of years and more, thanks to the Cage and his fight throughout time with Lucifer.
Nick can keep up half the time- he feels old, too, and thanks to Lucifer's possession he has all of the memories of Sam's own possession and the Cage, too, but that's always soured by Lucifer's presence, by Nick's own inability to differentiate himself from the Archangel, which makes him unpredictable.
Otherwise, Sam isn't always good with Jessica on his own. Mentally, Jess is only 21 years old, with snippets of memories from her dreams, from a timeline that has long since rotted, haunting her, but even with all her combined memories from her soul drifting from place to place, she's still only 24 at most.
When he took her form back in Georgia and anywhere else, Lucifer may have aged her up slightly to mirror Sam's own biological age- always frozen by Lucifer when he was out, so that Sam never felt human, so he knew exactly how much his body was under Lucifer's control, so he knew how his death and life were all in his hands- but Sam still can't quite deal with seeing her. Not after every time he burned with Lucifer inside the Cage while he wore her likeness, not after everything that body had done to him. It's her youth that gets to him the most, though, mostly because seeing her like this made Sam remember his own youth, how Lucifer played him and how his innocence and joy and freedom were just ripped away to be dangled in front of him on a string, and always reminds Sam of his own body and how Lucifer used his own helplessness against him. It reminded Sam of times he couldn't quite deal with yet, all the ways Lucifer made him love him and how he still had to fight that, every day.
Jess reminds him of all the ways they had been children set out for slaughter, reminds Sam of all the ways he himself hasn't healed yet, of his own vulnerability, of Sam being small and defenseless, of all the times he did not fit inside his own skin.
Seeing Jess reminds Sam of Jack, too. Jess died so young, having been manipulated throughout her childhood by Lucifer similar to how Jack would've been, if Lucifer hadn't jumped the shark and was forced to reveal his colors too early.
It's easier for Sam to deal with Nick.
Jess reminds Sam of how close he'd been to losing his son. How Lucifer nearly had everything and of all the ways Sam failed to protect Jack, because there was no way to possibly succeed.
And that's the last thing he needs to think about when trying to help the other people that Lucifer used, particularly when he needs to be there for Jack, too.
Nick is easier to deal with, oddly enough. Lucifer had hidden more of his true nature when he wore Jess to win Sam over. With Nick, things had been more straightforward, the mindgames more brute force and persuasion meant to force Sam to love Lucifer the way he wanted. And while Sam had more memories of Lucifer using Nick's form to hurt him overall, Sam had been older when it happened, had grown old inside the Cage, and even in the beginning, Sam had some of the wool ripped from his eyes, having already lost a part of himself to resignation and shame towards his own failings thanks to the weight of opening the Cage by the time Lucifer took Nick's form.
Sam's weakness, and knowledge of his own weakness, had made Lucifer's manipulations easier to fight. Because throughout it all, whenever Sam was at his lowest- when Sam only wished to give Lucifer what he wanted, wanted to make Lucifer happy so the pain would end, wanted to make Lucifer happy because he still loved him and just wanted Lucifer to love him back in an unselfish way that he never would again- but Sam didn't have to give in to his own weakness, and that made it easier to fight Lucifer. By not giving himself what he knew Lucifer had wanted him to need in the first place, Sam could use his own self-sacrifice as a shield. Sam could pretend his selfishness was the enemy, and not blame Lucifer at all for what Lucifer had done, and it drove Lucifer crazy when Sam shouldered the blame and pretended his love was the enemy. Lucifer knew he had no real way to fight that. More than that, there were so many memories of Lucifer not bothering to hide what he was when he wore Nick, and that was easier to fight, for Sam to keep his head above water and to avoid the things Lucifer had trained Sam to feel, for all the ways he had trained Sam to doubt himself and love Lucifer and make any false kindness feel safe or like relief. There was rules, and order, and a twisted, horrific familiarity to it, but at least Lucifer had been somewhat predictable, in a malicious, basic way that Sam knew how to endure.
The exception is when Nick is too familiar. Too much like Lucifer, when Sam can feel the echoes of the grace inside both of them resonating, and it scares Sam half to death, and those days, he can't be the one to look after him, not for a second.
All in all, it's easier for Sam not to push them all too hard. Not when Jess is still so damn young and still trying to get through college and trying to fight her way to freedom, and not when Nick is barely aware of who he is and so angry at himself and so empty, and not when Sam is still trying to feel safe, like Lucifer is finally gone, and when Jack is trying to move on, too.
Sometimes, Jack helps handle him. Sam makes sure he's in a safe enough headspace with Mia and that Nick can't hurt him in any way before they try.
But if dealing with Nick helps all of them, Sam won't be a hypocrite.
When he's himself, Nick likes Jack, even if he can't bear to deal with him sometimes because of old grief never healed.
He misses his family.
And sometimes, the disconnect from how he thinks of his own son, and how Lucifer thought of Jack, is enough for him to refuse Jack entry on principle, because of all the different feelings eating him alive.
Sometimes, Nick still feels the ghost of Lucifer inside him, feels the loss of his identity so keenly he latches on to memories far older than he ever would be, and if not restrained he would attack Jack and Sam on sight.
Those days, Sam has to keep him locked up as he tries to talk him back to himself.
Sometimes, Nick is subdued, mumbling, barely moving. Apologetic.
Unable to look at Sam head-on.
Other days he's angry, agitated, drowning in memories that are so hazy, he's not sure what is Lucifer and what is him.
"It will get better." Sam tries to be reassuring as he prepares the next bandage for Nick's wounded body, thanks to the last attempt Nick made to hurt himself. Today, Nick's more rigid and jumping at the bit, and more than once, Sam's had to leave the room to collect his thoughts and catch his breath.
"You act like it's so easy!" Nick growls, pacing.
Then Nick pauses and curls in on himself, notices his own elevated heartbeat and his hands in his hair (just like Lucifer, when he got frustrated), and the ragged breaths of losing his cool and how still Sam had gone.
The breathing is the only thing keeping Sam from panicking, because when he was angry, really angry, Lucifer didn't breath at all.
Sam closes his eyes. Inhales. Counts to ten. Exhales. Opens them.
"It's not." Sam whispers. "You know I know it's not."
Sam manages to approach Nick, and dabs his wound in silence. Nick braces for the pain this time and doesn't cry out.
(When he does, Sam always flinches.)
Still. Nick's thoughts whir, jagged and angry. Perfect saintly fucking Sam!
Nick hates him for his patience and his kindness and his understanding, more often than not, because it reminds him all the ways Sam had been strong and good and how he wasn't. How he just rolled over. How Lucifer made him feel invincible and how the anger felt so damn good and how he liked it, how much he learned to love Sam, too-
Then Nick is thinking of the Cage, thinking of all the things that it feels like his hands have done, and loses time again.
Sam manages to tap him other the shoulder, trying not to tremble at having to reach out, or at the bare skin.
"Nick, you with me?" Sam prompts.
When Nick is aware of himself, when he can delineate Lucifer's memories from himself, his body language is always sluggish and closed off, and how he talks, when emotion doesn't overtake him- it's different from Lucifer entirely. And Lucifer would always say Sam's name, nickname or otherwise, because he liked to feel like he owned it. Nick, when he's aware, never says it at all.
Sam holds on to those attempts, where Nick tries his best, but they both know it's few and far between. Sometimes, it's all too familiar, and they're both too familiar with each other, both feeling the ghost of someone who isn't supposed to be there, like they aren't even talking to each other.
"Why are you even bothering?" Nick asks, voice wavering. He almost grasps Sam's wrists and then realizes what he's doing and stops, because the last thing Sam needs is contact, is the muscle memory of how Lucifer-
Nick remains frozen, head bowed, scared to look Sam in the face.
Somewhere down the line, he thinks learned to love him, that he learned to love them both, with Lucifer's obsession bleeding through, so all-consuming that all Nick wanted was to feel it and have it obliterate every other feeling, because Lucifer was so sure, and so confident, that Nick couldn't feel any pain or regret or hurt at all.
"Because you are my friend, and we're in this together." Sam assures, finishing up with the antiseptic and bandages.
"I don't deserve-" Nick tries, but Sam doesn't let him finish the thought.
"What he did wasn't on us, Nick. And even if it was... It wasn't you." Sam answers with finality.
But Nick knows it was. He knows it.
He doesn't know who he is, otherwise. And it feels like a betrayal, to think anything else. Nick thinks, sometimes, that Lucifer loved him. Not like he loved Sam, not at all. But loved him in the way that a vassal was useful to it's master, and he loved him like he would a pet- because he knew he owned Nick. Knew Nick couldn't fight him at all, and that Nick understood him, empathized with him and his frustration and his rage and need to destroy, because all Lucifer wanted was to be happy and if he was happy Nick would be happy and they could make Sam happy except-
Except Sam ruined it. Sam took mercy on the world that Nick still wanted to rip apart, because if Lucifer let them rip the world apart it would even the scales, somehow.
It made Nick feel untouchable.
Just like it had made Sam feel whole, even if hurt them, too.
Sam walks out, everything having caught up to him now, and with his mind still able to pick up on some things, with Nick-
He can't stay.
Sam is unable to say much more except that someone will bring dinner if Nick doesn't feel like joining them in the kitchen tonight. Sam can deal with Jess and Nick in crowds, provided he's with the rest of the family. That's always been easier, although it's rare, seeing as Nick doesn't like being around a lot of people.
As Sam leaves, Nick watching he just feels lost, and empty, and lonely, and that feels like Lucifer at his lowest, too.
Nick hates both of them for that.
He hates both of them for leaving.
A week before Nick leaves, there's more than one incident.
Sam kicks himself for not seeing the warning signs, but Nick backslides often, and Sam's more withdrawn.
His dreams have come back.
He's scared to fucking death, and keeping busy to try and hide from it.
But Sam can't hide it from Nick. Nick always reads him like a book, because Lucifer could, too.
Somehow they end up arguing about something entirely different.
Sam storms out, and goes to find Jack, goes to try and ease his troubled mind.
Nick trashes the room.
The day before Nick goes missing, their argument ends with something else.
"I don't know who I am without him!" Nick explodes, leaping up and pushing Sam into the wall. "And I don't think you do, either! So tell me, Sam! How do you deal with it? Huh? Because I don't think you do. I think you're lying just like the rest of us, pretending every is fine..." Nick trails off. Somehow he's gripped Sam by the throat.
Sam's breath hitches, tendons bobbing under Nick's palm. He stands frozen, muscle memory kicking in, because all that rage is all too familiar, and Sam thinks of everything Lucifer used to be-
Then Nick's eyes turn glassy and he just lets go, Sam taking a few collapsing breaths, and Nick puts head in his hands before he curls up on the bed.
"You should go, Sam."
Sam stays stuck to the wall for a moment, not daring to move.
Nick said his name, said it more than once, and it sounds just like him-
Sam tries to keep his footsteps steady as he tries to make his way to the door without looking like he's fleeing.
Nick never once looks Sam in the eye the entire time.
Sam wishes he would when he takes one glance back.
Because at least, if he did, Sam knows he won't see Lucifer inside them.