I wonder about this, too. My AW often talks to her mother on the phone while drunk, and I can't imagine how her mom doesn't notice. I really don't think she knows, or maybe she thinks it's some of the medicine AW takes. If she suspected, I think she'd ask me about it. AW's sister is extremely religious, and would be mortified by her drinking.
If we ever divorce, It may become necessary to tell them, because I'm sure I'll be portrayed as the villain. But then, maybe it would be better just to leave it alone. They'll eventually figure it out when AW ends up in the hospital from an alcohol related illness.