Definitely keep in touch with DiL & GC, but try to stay neutral so as not to rock the boat.
Your son is paying for a roof over his children's heads first & foremost, not hers! If they own the house he could have it put in his will that should anything happen to him, his half of the house goes to the children. If it's sold, his half goes to the children at a suitable age!
I know it's hard not to interfere, even in what we consider the best interests of the children, but try not to, for the sake of your relationship with them and their parents!!
(If she moves someone in, they should definitely pay rent).