Reading this again, BabyLayla - how lucky XDIL is to have a XMIL who appreciates and cares about her so much! She may be very busy, though, as you say, especially adjusting to her new life. Or she just may not be ready to be around anyone who reminds her of DS right now, sadly, not even a loving XMIL/you. Or she may feel she has to loosen the ties since she hopes to meet someone new eventually. Or... no way to really know. If she doesn't accept your offer to come to dinner or just doesn't respond, please don't take it personally. It's not about you, surely, but about where she is mentally and emotionally right now.
Glad you got to talk to one of the GC, and that XDIL isn't trying to prevent that! Hope you continue to have a relationship w/ them even if XDIL, herself, pulls away. And, no doubt, DS can facilitate your relationship w/ the GC too. (I realize you can't expect too much now, concerning the GC, since they are older and busy w/ their own teenage lives.)