pretty sure it's because there's a lot of stigma against foxes, and I'm pretty sure he's also a half-breed, which makes it worse. and she's traditional, and really wants the yeseung (the big bro/alpha/idk his name) to marry another wolf
I don't think she really hates him, she just feels sympathy for him but its not like she doesn't feel some hatred towards him either. the wolf breed have a lot if pride in their heritage of peer blood so when he was born she felt Hurt by how her own husband cheated on her and also ruined their peer blood, but she also knows the amount of pain an innocent Child would go through if he stayed. But this is just my opinion. ╮(￣▽￣)╭
Can someone remind me why the mother doesn't like the fox? If it's because he's an illegitimate child, shouldn't she be wise enough to know that it was her husband's fault not the fox? It's been so long since the beginning of the book.