The first time me and my A split up a mutual friend asked his sister if she had spoken to me. Her response was "I have no reason to speak to her now". Says it all doesnt it? and to a certain extent, true, what does she owe me? The second time we split up he did his usual and ran back to his mum (who I was meant to have a good relationship with) and she didn't bother to text or call to see if I was ok for over a month. In the end I called her because I was so gob smacked that even after all the support etc I'd given my partner (that she had seen), when it came down to it, blood really was thicker than water. As it usually is.
Those are just two examples (I've got loads) of their behaviour that led me to change the way I am with them. I'm not rude but I don't go out of my way. In fact I haven't attended a family function now (his family) for nearly 2 years now. I also keep in my mind that they knew him for a lot longer than me. They had every opportunity to challenge him about his drinking and have had opportunities since I've been with him. They didn't and I know that even if they did it would make very little difference anyway. It's easier for them not to.
I really think you should focus that energy on your friends and family when he is mixing with his. I do that and it really helps me.