How wonderful for you riciorian! How kind of you, too, to share this note of hope with others! I'm not estranged from my GC, fortunately, but I know some GPs who are. They would see this as a good omen. I hope you don't mind if I tell them about it.
Obviously, DIL has issues with her sons now, the same way she did with you. Perhaps the same is true with your son, even if he just goes along with her. It's great that you're able to help the older GS! What loving GPs you must be!
It's odd though that the younger one believes you and DH over his parents. I'm sure you're telling the truth and they haven't been, but why would he think that? He must have had other experiences with their lying to him.
I have to admit I'm sorry your GSs are in such conflict with their parents. However, I'm glad they feel they can turn to you and DH and that it all led to this joyous reconciliation!