Sometimes, Lily wondered what her sister and her sister's family might have been like without her. And somedays, the thought made her shudder.
She didn't have to ask why Petunia married Vernon Dursley. In his youth, he hadn't actually been bad looking, and most importantly he adored Petunia unconditionally and passionately, and was probably the very first person in Petunia's life who ever did. For that, Lily was willing to put up with him, even if he was otherwise a fairly horrid person.
No, that wasn't fair. Vernon wasn't a horrid person. Just a difficult person, with absolutely no imagination, a poor sense of empathy, far too much assurance that he was always in the right, and enough prejudices that he could be relied on to pull one out for every occasion. If he'd been a wizard, Lily often thought, with a little bit of venom, he wouldn't have been a Death Eater. Oh no; he wasn't that bad, and part of what he prided himself on was not being that bad. He had a sense of morals. But he certainly would have been one of the idiots at the back of the room when Lucius Malfoy was making his oh-so-reasonable speeches about pure-blood superiority, shouting "right!" at every pause.
In short, Vernon Dursley wasn't a bad person. He was just the sort of person who let bad people get away with some of the things they did.
But he loved Petunia and genuinely wanted her to be happy, and in almost all things made damn sure she was, to the best of his ability. And he doted on Dudley, and, Lily was actually quite sure, would have taken an Unforgivable Curse for his wife or his son.
And, finally, he did think, as long as you forced him to. That just took a great deal of effort. Or, in Lily's case, a great deal of ignoring his complaints and pointed comments, which was generally as far as he dared take it, being more than a little afraid of her. It just so happened that Lily felt fairly certain that if she hadn't been around, nobody would have bothered to force him to do that thinking.
Petunia certainly wouldn't have, not without Lily around to make her feel guilty when Vernon said something horrible or bigoted. Not because Tuney didn't know better, maybe, but because Tuney didn't want to know better: it was work, and sometimes it hurt, and Tuney still had a big pile of resentment against the world. Some days, Lily felt more sympathy for that than others; those were the days when she made herself remember that being the plain girl in school who didn't really have any particular talents and, at the time, was painfully shy wasn't very much fun, especially (Lily knew) when your sister had gone away and made a hundred friends and continually brought raptures of pride from your parents. Lily, in retrospect, was more than a little angry at their parents for that: you weren't supposed to have favourites, and comparing one child to another was simply cruel. They should have found Tuney something to be good at, something for them to coo over her for, but they didn't.
Petunia didn't have friends of her own, at least not before Lily collapsed on her doorstep. All her friends had been wives or girlfriends of Vernon's friends. And in that little echo-chamber . . . Lily sometimes shuddered at the thought of what her sister might have become, and even more at how miserable, petty, and vicious Tuney might have turned.
Other days, when everything overwhelmed her, Lily berated herself for thinking she'd have that much of an effect on anybody, except possibly Harry. Then she thought of Harry growing up without her and it was a thought to give her nightmares. If Dumbledore's speculations about why Harry had survived at all were right, he'd still be alive if Lily were dead.
Alive, and completely alone.
Whenever she had that thought, she found herself finding him, wherever he was, to give him the tightest hug she could. Harry took that, depending on the day, or what he was doing, either with good grace or with the annoyed impatience of a boy whose mother is interrupting him from doing something important.
Today, five days before Harry's eleventh birthday, when he'd just patiently let her hug and kiss him and then send him away to read one of his books, Lily sat on her sister's couch and hugged her knees - both of them, the plastic-metal-and-whatever and the flesh-and-bone - to her chest. When her sister came in from the kitchen with her coffee and her crossword, Lily looked up at said, "Thank you, Tuney."
Petunia looked startled, and then suspicious. "For what?" she asked, obviously trying to modulate her tone about halfway through into something less sharp. Her new haircut, Lily thought, really suited her. It was short and it made her look strong and refined, turned her slightly raw-boned features into something with dignity. Lily would forgive that shallow harridan Ruth Harvey quite a lot, for talking Petunia into that haircut.
"For having us," Lily said. "For letting us stay here, all these years. For everything."
For a moment, Petunia looked like she was actually flustered. Then she waved her hand and said, "Honestly, Lily. It isn't like I was going to throw you out into the world or anything, what must you think of me?"
I think it was really hard for you to do, Lily thought, but didn't say. I think some days you regret it a lot. I think you've had to do a lot of work on yourself to be able to stand me living here, and not to spend all your time terrified that Harry's going to grow up to be more important - and so more worthwhile, in your head - than Dudley is. Out loud, she just said, "Just because it's something you'd do naturally doesn't mean I can't be grateful, Tuney."
Petunia glanced down at her crossword, and then up at Lily and said, with feigned briskness, "You're my sister, and you're very welcome." Then, with an air of firmness, she propped her crossword book on the arm of the couch and took the cap off her pen.