Re: daughter.in.Law, Son won't see
The thing here is that your son may be in deep denial.
When emotions are involved,they can often trump intellect.
It seems that your daughter in law has issues but if neither your son(her husband) or her want to address the "elephant in the room" then there's not much that can be done on your part.
Be supportive of him and be there if the chips eventually fall(which wouldn't be a surprise).
Above all else,please don't resort to violence of any type;it only makes a bad situation worse.
When in doubt, post it out.