Simply speaking from my own experiences, both dealing with G's drinking, and even my parents growing up (not drunks, just narcissistic), it's very hard to admit that someone close to you has a problem; even harder to admit it's a relative. He probably got to them and told them his side of the story, and they either believe it or are sticking by him because "blood is thicker than water".
I'd suggest to keep in mind that right now, they're going through a similar process to what you went through before deciding to leave. You may never be as close to them as you were before - either because they believe him or because they feel it would be awkward to continue to "keep in touch". (G's youngest son is divorced and remarried, and his son's ex-wife still tries to contact him on a regular basis. G is conflicted, because he understands her side of the situation, but doesn't want to complicate things by opening up any further communication with her - even though his son and his son's new wife haven't spoken to him since the wedding - and he actually found out about his new grandchild on the way from a mutual acquaintance asking if he was excited).
It's in their hands, regarding what they believe and what actions they're most comfortable with, and has nothing personal to do with you.