I think I'd give him the hardline. I would tell him that I need his support and that his family will never accept me unless he takes me more seriously and at the very least explains how I am, if not actually sticking up for me.
I feel like, part of the problem here is that he is not doing anything to fix the situation. Personally, I would try to make him understand how important it is, and if he cant see it, then just break up with him. He should have faith in you, and have respect for you and he shouldnt let his family treat you badly.
On the other side of things, he IS still related to his brother and unfortunately his skankface ho baggish sister in law, so you cant expect him to never see them, even if you would like that..
But I do think he should stick up for you. Cheer up chicken, it will all sort itself out eventually