Geez, that's a tough one. Do you have a relationship at all with either of the two boys? Can you help with putting up the younger one and his girlfriend and "counseling" them/helping them to get jobs? I think if I were squeezed in there (the kids) that I'd want a little room so if you could offer a basement or some place to make their own for a bit with clear rules and "help" of some sort, that could be an incentive?

If it gets too chaotic and nothing is working, I'd call social services to see about the very young kids. Do you know your daughter-in-law's parents; could they be of any help uniting with you?