I've seen some say that Touya resents her because she's unable to stop Endeavour, which isn't uncommon in children to displace their anger onto other family members who fail to stop abuse, but it's shown Toya still idolizes Endeavour and is even offended Rei suggests he shouldn't

I think his resentment in part stems from the fact that her genes are the ones stopping him from being able to use his quirk. He calls her good for nothing, and resents she passed that perceived uselessness to him, which is why he shows a total lack of respect or love towards her

I think he also views her as weak, since, from his perspective, she had no control over what her family did with her, Endeavour easily overrides her in their relationship, and she can't even manage to control or parent him.

He kept looking

LET HIM JOIN THE FAMILY ALREADY.