My other half is the same, dislikes his own family. I told him to stop being so ungrateful and stop acting like a little spoiled s***. I remind him of the good things they do for him. That usually diffuses the situation and lets him consider the positive side of them.
However if he does start having a go at me about it, I just tell him to deal with it cos I cant be asked to listen (as well as really don't care) which usually leads to him having a little strop and then apologising afterwards.
He is a lot more tolerant with them now with the exception of his sister. I think she screwed him over a few times due to her ditzy/forgetfulness so he'll never forgive her for that.
Don't know if this helps but it worked for me.