Stannis never knows what to say to women. His lady mother died when he was just a boy, and the nursemaid who’d helped care for him even before that, so he hasn't had much interaction with women. He's serious and proper and not amused by their antics, and so the serving girls have learned not to giggle in his presence and offer filthy favors.
The castle is a different place when Robert comes home for one of his infrequent visits. The hall is much more boisterous. There’s too much noise and unruly behavior for Stannis’s liking, but Robert is his elder and the lord of Storm’s End, so he holds his tongue and endures the serving women forgetting their place and crowding around the high table.
“A freshly bloomed flower,” Robert booms, stroking the blushing cheek of one girl. “How have you let this one go unplucked, brother?”
“I do not debase myself with serving wenches.”
Robert roars with laughter and mocks him for an "old maid," and perhaps a weaker man might have bedded a woman to prove his manhood. Stannis just grits his teeth and tells Robert that House Baratheon needs no more bastards. Robert is unashamed - he just laughs and claps Stannis on the back, and downs another cup of wine. Then he pulls the girl into his lap.
Only a few short months later Robert's betrothed Lyanna Stark is abducted by Prince Rhaegar - at least according to Robert, the smallfolk say the Stark girl eloped with the dragon prince - and Stannis finds himself awkwardly trying to console a brother who's already comforting himself with wine and a pair of whores.
He thinks long and hard before agreeing to hold Storm's End - much to Robert's annoyance. True, Robert is his brother, and true the crown prince is already married with a small child and should not be stealing the fiancee of a great lord. But what Robert is suggesting is open rebellion against the lawful king.
Stannis remembers how impressive the king had looked seated upon the Iron Throne - but then he remembers that he'd seen King Aerys's Hand Tywin Lannister, not Aerys Targaryen himself. Perhaps Aerys just was not meant to be king. He's never heard any good said of Aerys; they call him the Mad King with good reason.
He thinks of his lord father and wonders what Lord Steffon would have done in his place. He isn't sure, but he does know how important family was to Lord Steffon. Before he and Lady Cassana had left for that fateful journey that would cost them their lives - an errand tasked by Aerys, he remembers - Lord Steffon had gathered his three sons and made Stannis and Robert promise to take care of each other and baby Renly while he was gone. So he says yes and assumes command of the ancestral castle - surviving an inglorious siege - while Robert rides off to earn glory.
“Your brother means to bring his little rose’s sister to court,” Cersei says to him one evening. Her tone is one of complaint and her face is fixed in displeasure, but when isn’t it?
“Mayhaps he means to take the girl to wife,” Robert replies idly. Little Tommen is showing him a drawing of a cat armored like a knight. Robert pats him on the head, before gulping down another big sip of sour Dornish red.
Cersei laughs shrilly. “Renly take a wife?”
“He’ll need to sow seed on fertile ground to get sons.”
“More like he means to plant the girl in your bed. I warn you, I will not…”
“Warn me?” Robert roars. He hates that his wife is accusing him of plowing some girl he has not yet even met, and he hates that she’s right. He hates her too. If she wasn’t so cold, if she was his Lyanna… “You are warning me, woman?!”
The nursemaid hurriedly hustles the younger children from the room, and the Hound leads Joff away. Only the Kingslayer remains with him and Cersei, with a look on his face that tempts Robert to smash it in.
Mayhaps Cersei senses it. She orders her brother, “Leave us, Jaime.”
“You forget which of us is the king,” Robert says to his wife. But if he’s honest, his anger is not for her alone. Seven, but he wishes some lord had the balls to rebel like Greyjoy had. He could use a good honest battle. That’s what he was made for, not for this bickering and politicking.
“You are the king, Robert, and our son Joffrey will be king after you. That is his right. Do you want some upstart bastard stealing his throne from him? Have you forgotten what Aegon the Unworthy’s bastards did?”
How dare she liken him to the king maesters and singers alike had given the epithet ‘Unworthy’.
“Fuck all the whores and tavern wenches you want, but if you get another bastard on a highborn girl, I’ll see that neither she nor the bastard live.”
He doesn’t mean to hit her. It just happens. She makes him so angry. All he can do is leave before he strikes her again. He finds a flagon of strongwine and an agreeable serving wench and loses himself in them.
His head is aching from too much wine the next day, but he hasn’t forgotten the night’s conversation. He seeks out Stannis, and, to his surprise, finds him breaking his fast with Jon Arryn.
“I have a mind to betroth Renly to Mace Tyrell’s girl,” Robert announces. “What say you?”
He is the eldest brother, as well as Renly’s king, but Stannis spent his own youth raising Renly, so it only seems right and proper to speak with him about the matter.
He doesn’t expect Stannis to agree immediately - Stannis wouldn’t be Stannis if he didn’t find something to complain about - but he’s surprised to see him exchange a look with Jon.
It’s Jon who speaks. “It could be a good match, Robert. But perhaps now is not the best time for such things. If you wait a little…”
Stannis gives a curt nod.
So. The two of them have been discussing matters they do not care to make him privy to. Robert can’t help the bitter pang of jealousy and resentment. Jon loves him like a son, but this is proof of what he’s suspected for some time: that Jon doesn’t like him very much anymore. Stannis - dour, boring Stannis, who lives like an aged septon - is the sort of man Jon would rather have as a confidante.
“Ah, fuck it,” Robert says, throwing up his hands. “I’m going hunting. Do as you will.”
Of course, Stannis is like to alienate his allies before very long. And Joffrey’s sixteenth birthday is only a few short years away. Renly is much better suited to court life and politics than Stannis.
Renly has no claim to the Iron Throne with Robert having two trueborn sons and a brother older than Renly. But everyone comments how much Renly looks like Robert when he was young, and Renly is good at charming lords and smallfolk alike. Robert hadn’t had a claim to the Iron Throne, but he’d taken it nonetheless. Renly can take the Iron Throne for himself.
He’ll do it. He’ll become king. Stannis will be furious - his brother takes their House words much too seriously, in Renly’s opinion - but Renly will make him lord of Storm’s End. He knows how much Stannis wants it. Stannis will forgive him eventually.