The author has kept this arc going for quite a while without the tension break of actually learning what the problem is that's causing this negative reaction/tantrum. I have my guesses, presumably something related to all the bother she made for her adoptive parents and feeling very guilty and not wanting them to go through that with another brat? But Xiaoyang really doesn't seem that bad, whatever trouble he causes can't be worse than what Xiaoen is doing with her tantrums and enbyphobia. It's obviously a very irrational position she's in. Definitely appreciate Xingyuan countering her trying to say some vile shit she probably doesn't mean just to hurt Xiaoyang and chase him away.
I think we do know. Xiaoen was the unwanted child, given up and forgotten/erased from her real parents. Xiaoyang was her replacement as the son, but Xiaoyang wasn't who the parents wanted them to be despite the expectations and possibly a façade. Xiaoyang then tries to be who they want to be outside the family, repeatedly going to Xiaoen who is basically traumatized (for lack of better words, even I think trauma is a bit too much). So whenever Xiaoyang is around, she's reminded every time how people outside Peachtown basically don't want her.