Disclaimer: Lost doesn't belong to me, and I make no money off of this.
When she had been younger, Shannon had had nightmares. Boone knew this because afterwards, she'd always come to his room and woken him with nudges and desperate, tearful whispering, and he'd always moved to the very edge of his bed and let her sleep beside him. It was something he'd accepted as yet another duty that came along with being an elder brother, as Shannon would never get close enough to Sabrina to let it be a duty that came along with being a parent.
He didn't mind, though, because the shivering girl curled up beside him at night wasn't the same as the princess--and even at ten Boone knew that princess was just a nicer word for bitch--that flounced around by day.
When she was older, Shannon still sought out Boone to rescue her from nightmares, but now they were of the daylight sort: boyfriends that hadn't worked out and parties where she'd had too much to drink. By now it was more like habit than newfound duty. He told himself that he didn't have to do it, that she could take care of herself, but this was a lie. He'd ignored her calls for an hour once, listening to her messages grow more tearful and more desperate, and he'd mumbled excuses for his tardiness when he'd picked her up.
Sometimes she'd shared his bed after these nightmares as well, but it was never innocent and never chaste and he suspected that it wasn't for comfort so much as it was just another way of her being a bitch.
Boone knew that Shannon had nightmares on the Island as well. This is both common sense--Who wouldn't, in their situation?--and because he'd seen her stir in her sleep and heard her cry out softly in fear. He would stroke her hair until she slept peacefully again, and he'd only mentioned it once.
"God, that must have been some kind of nightmare you were having last night."
She had been in a particularly bad mood that morning, and had snapped, "What do you mean, nightmares? I had a dream that you died. How is that a nightmare?"
He bitterly assumed that it would have been different had it been Sayid commenting on her dreams, which wasn't at all fair, because he was the one she'd been running to since she was eight, and Sayid could never say the same.
When he was with Locke, Boone had the first nightmare he can remember that's ever sent him running to someone. Like the worst of dreams, it seemed tangible. He felt the limp weight of her body in his arms, feeling at the same time the sick relief of a burden lifted. When Locke told him that it wasn't real--an oddly disgusting perversion of the words he'd used to soothe Shannon so many times--he had to reassure himself by watching her obliviously flirting with Sayid, still coquettish and ungrateful and very much alive.
He almost wished that she had an obligation similar to his, to calm and console in the half-mad moment of waking, but Shannon was never so responsible, and it's another duty of elder brothers to console and without needing consolation.
He turned and left her, but he didn't expect to be gone for long, because she had nightmares on the Island, and he never could ignore her calls.