Welbeck you seem to think that the motive you suggest is heinous, they are the OP's grandchildren, so why shouldn't their mother protect any inheritance?
But once again, does it matter. If it ensures harmony and a good relationship between ex DiL, grandchildern and their father's family. Whats not to like?
Trump tears peace hopes into pieces.
What’s a household item that reminds you of your grandma’s house?
I would focus on the fact that improved contact with your former d.i.l. will make it easier for you to retain your relationship with your GC, as that will not be just at the behest of your son.