I'm sure you have their best interests at heart and don't want to see him manipulate and use them but he's already doing it, isn't he? They are allowing him to do it. JMO, but if I was dealing with my in-denial in-laws, I'd allow them to go through it and really experience it first-hand. I think that's the only way they will get it.
As far as him mom's concerned, she wants him around and his threat of walking out and not coming back is his trump card to keep her in line. Even if they were fully aware of what they're up against, what good would it do? He'll continue to drink regardless of what they know. They'll continue to act out their denial and codie behaviors until it gets too painful for them.
Love them and pray for them ... but perhaps it is better to do so at a distance.