free2be - do you remember how you felt and how you acted back when you were with your STBAH? If someone would have come to you and talked to you - would you have really listened?
You can talk to his family if you want too - you can tell them the honest truth and try to get them to see the light. But don't hope too much that they "get it". They love their son and they haven't come to the point where they can pull their heads out of denial enough to see/admit that he has a problem. And until they are ready - they just aren't going to do it.
I'd say that it's really up to you if you want to talk to them - but I fear that you will get hurt. They may actually turn on you thinking that you are just slamming and lying about him. They will see the truth on their own - but only if they choose to open their eyes and see what's in front of them.
As with the A - we must all find our own reality - when we are ready.
(P.S. My XAH's parents are in denial as well. They enable my XAH too. Funny thing is that they have admitted in the past that XAH has a problem - but to this day - they are still doing the same old thing - enabling him. It's their choice.)