Standard disclaimer here.
He watches her from far away, struggling to hold onto the idea that she is his sister. Her laughter is poison, he knows, and it kills him to watch her laugh with that… that… him. Not him. Someone else. Anyone else.
It's like a broken record, he thinks sourly, eventually she will just get her heart broken, and then what? Big brother to the rescue. He does it because he loves her, does it because he knows she needs him, does it because she's like a drug and he can't live without her.
Every girlfriend he's ever had was a blonde. Always her same dress size, too, because he knows his sister won't miss most of the skirts he pulls from her closet—knows she won't even notice, probably. The girl (what was her name again?) would twist and swirl and dance and laugh and moan, and it would be so easy to envision his sister in her place, so he does. It's not the same, oh god no, but it's close enough so he takes it.
He dreams of her dying in his arms, body a bloody and mangled mess, and feels relieved. Sad, yes, but mostly relieved, and indescribably free; something he hasn't felt for a long, long time.
Sometimes he dreams of a pretty freckled girl, with light hair and a nervous smile ('Hi…' she says shyly, blushing under his gaze, 'My name is Shannon…'), who can't breathe so well, and gets embarrassed easily. He wonders when she—him too, maybe they're both crazy—became so self-destructive.
He wonders why he loves her so much, even when it's clear that she doesn't love him, never has, and never will.
(even though she slept with him, the traitorous voice in his mind whispers, even though she wrapped her legs around his waist and cried his name and let him touch her—even though, in the height of passion, she told him she did; she didn't remember saying it later, but he does and doesn't think he'll ever forget)
Boone watches his stepsister from far away.