Like in the summary it says, this is my take on why Tommy has superspeed while Billy has the power to alter reality

Disclaimer: I do not own Young Avengers

"…I hate this power…" Thomas mused softly as he stared at the spot where the man in purple once stood. He hadn't meant to erase him from existence; it had been a complete accident. This power was too strong, for him and for his mother. Only his twin brother ever seemed able to get a grasp at controlling the life force. The twin that was currently looking out of the gaping hole that used to be a wall in their room. Their mother had destroyed it in her frenzy to get to her brother's side, abandoning her sons in the tower to fend for themselves after Doctor Strange hastily followed her out.

"Really? Why?" William asked, not turning away.

"It scary and it makes Mommy sad." Thomas replied as he made his way over to stand beside his brother. Below them, almost unable to be made out, their mother just subconscious removed their grandfather's mouth in her desire for him to listen to her for once. The pain she felt, the sorrow, the anguish, all of it flowed into her sons through a connection she didn't know existed. But the twins knew. They knew everything; about the other life, about the other them. "She's going to set it back, you know"

"Yeah she is. "

"What's going to happen to us?"

"We'll go back to."

"…will we remember?"

"I don't think so"

"…"

"Still scared?"

"…yes…"

"Then I'll take it all next time. " William mused as he watched his mother's body shake with her sobs, with all the words she had been holding in. She looked so small from up here, so lonely. He promised himself quietly that he would find her when everything went back, that he would save her.

"But what if you become like Mommy?" Thomas asked, worried. He loved his mother as much as his brother did, but he couldn't deny her health issues. " What if you try to destroy everything?"

This gave the younger twin reason to pause. He looked down at his silver haired uncle, his mother's own twin, the only person who could ever calm her down. He had always been the anchor of Wanda, was always the one to whisk her away when it looked like she was going to break.

"…than I guess you'll have to be fast enough to stop me." William said with smile on his chubby face, he held out his hand for his brother. "Deal?"

Thomas smiled back in understanding, taking the hand as a white light blinded them. "Deal."