Fear
Twins were supposed to have a special bond. They were supposed to be inseparable, two halves of the same whole, two people with connected minds. Twins were, in fact, supposed to be everything that he and Wanda weren't.
He was trying. She didn't understand just how hard it was for him to act as though she was the sister that he had loved, that he had indeed had that special bond with, before she had gained her powers. It wasn't her fault. He'd become very good at hiding what he felt, so good that no one knew what he was thinking unless he wanted them to, which wasn't very often.
The moment she walked in the door he had lost it, calling her name as if in a dream, begging for it not be real. His request had been denied with a force that sent his mind reeling and his insides churning with the same raw terror he remembered so well from before. He had always been the weaker twin, the one that went along with whoever was in charge, who didn't have the guts to stand up to his father or to anyone else. Before they had gotten their powers he had leaned on his sister, staying in her shadow and presenting a cheerful face to anyone who looked past her to see him. She had protected him, comforted him, ordered him around, basically been the stable one, the person in charge, for those long times when their father was gone, either physically or simply sunk so deep in his own mind that he didn't notice them anymore.
It had been his sister who had given him the strength to be an ordinary kid, and he knew that his dependence on her made her feel stronger and older, better than anyone else. Together they had been perfect, until that one day when everything fell apart. Before then there had been accidents, times when things around her would break and it seemed like Murphy's law was the only one that was still in effect, but it had always been ordinary things, really. People would trip over their own feet, the power would go out, he'd trip and fall, things like that. On that day, however, she lost control of her powers completely, either that or she'd intended to cause as much damage as she could.
The bruises hadn't really shown. His father had made sure that no one knew, saying that it would be a disgrace to their family, but Pietro had simply been too afraid to let anyone know that his sister, his wonderful older sister, had used her powers to fling everything in the room around, including him. She'd been angry at their father, something that happened almost every day back then, angry that he wasn't there more often when she wanted him, angry that he didn't care about them in the same way that everyone else's parents seemed to care, angry just because she was a teenager and she wanted to be. Pietro had tried to convince her as he always did that it was better to have a father who didn't care about you than not to have one at all, and she'd just been so angry that everything had fallen apart.
Pietro had silently agreed with his father that Wanda needed to be sent away to somewhere where she couldn't hurt anyone anymore. He'd loved his sister once, but after what she had done he was simply terrified, unable to think of anything when he looked at her other than what she could do - what she had already done - to him. It didn't matter by then that she hated him, that she would see his siding with his father as a betrayal. All that mattered was getting away from the terrible figure that had already begun to haunt his dreams.
He still wanted to get away from her. He knew that if he had been in her shoes he would have wanted to hurt himself for abandoning her, but that didn't change the fact that he was still terrified of his sister. Although someday they might be able to love each other again, he'd never forget what she'd done.