My heart goes out to you and dh, debohhun. How painful to lose your dd and now, to lose access to your gc on top of that! Hugs!
I think dh is right, though, and that you should wait till gc are old enough to reach out to you. Not only would a court case be costly, but there's no guarantee you would win. Nor is there anything they can do about sil being "awkward" around you.
Chances are, he moved away to try to start a new life. I imagine he chose to be near his parents and get their help because he feels more comfortable that way. If and when he brings a new woman into his life, he may not feel he should have his late wife's family around. I just hope, eventually, when he gets himself sorted, he'll let the gc reach out to you. Only time, of course, will tell.
I don't see why he would do any "damage," such as talking against you and dh, unless you pressure him for contact. So I think there's a strong chance the gc will look you up later on. I know it's a long time to wait, though, and it may not happen. Once again, so sorry.