I lurked on your last two threads but I am getting so annoyed, it's like they're trying to piss you off!
This.
I suspect that that is exactly what the prick is trying to do. He is controlling towards her and probably absolutely hates that she has a relationship with anyone but him. How better to solve that than engineer a blazing row with you over this, where your parents and siblings back you up, and then he can persuade her to cut you all off as you obviously dont care about her or her family?
Stick to your guns, but do keep the lines of communication open with your DSis for when she (hopefully) sees the light and wants rid of this tosser, she will need you then as he wont go quietly.