Here's my :cents:...

What happens between your mom-in-law, dad-in-law, and this guy is there own business....behind closed doors. When it starts making its way into public and family functions, well that's a different story.

If this guy is making an ass of himself, and feeling up mom in front of everyone (especially the kids!) then yes you and the rest of the family have every right to speak up. It's not only disrespectful to your father in law, but to everyone else. What kind of message are they sending to the kids that are around? That it's ok to treat women like that? Bullshit.

Granted it should be your father in laws place to say something, but for whatever reason he feels as though he shouldn't. I don't see anything wrong with the rest of the family saying for him though. Get the rest of the fam to back you up and then get on this guy.