Does not visiting oppressive relatives constitute severing ties of kinship, which prevents prayers from being answered?

It is permissible to supplicate against an oppressive relative to the extent of their oppression. Your supplication for Allah to protect you from their injustice is not an transgression, because the hadith that mentions the supplication of one who severs ties of kinship is not answered does not apply to your supplication. You have not supplicated for the severing of kinship ties. The meaning of the hadith is that a supplication for the severing of kinship itself is not answered, not all other supplications. You must not sever ties with your relative if they are someone whose kinship must be maintained, but you can switch to another type of maintaining ties that does not harm you.