I would never let myself in to DD's house when no one was there unless by express arrangement, or in an emergency, although I have a key. She wouldn't let herself into our house either, if we were out, because it would be us she came to see anyway. I know a lot of families have an open door policy, so to speak, but I've always been a very private person and like you, crimson, find it awkward if I have to 'entertain' any but close family members.
carol, now you've given us a bit more information I can see how you could feel excluded. If DIL's own family is welcome but your son's is not, that puts a different complexion on the problem. I would still say just enjoy the time you have with your grandchildren, though, and try not to show resentment towards your DIL. Feeling bitter will only make you feel worse.