Hi Canonlytry. I'm sorry you're disappointed in your relationship w/ your DILs. We MILs are forever being told to try to develop our relationship w/ DIL, but some DILs just don't seem to want it. I'm getting the impression that you hoped it would be different w/ your 2nd DIL. But I think it's time to stop trying to make that happen and just accept things as they are.
It often seems when this happens, that either DIL is very close w/ her own family, feels she has "enough" family and friends, or doesn't have a good relationship w/ her own mum (and so probably doesn't want one w/ DH's mum). If DIL is as busy as you say, she may prefer to spend her free time w/ her own family, friends, or even just relaxing on her own. Maybe she doesn't know DS visits you, but perhaps she's glad he doesn't push her to join him.
Looking back, I had a more or less friendly relationship w/ my MIL and some good times along the way. But we never went out, just the two of us, and I don't think either of us would have enjoyed that. We were only together when we visited w/ each other as a family, and we were both happy w/ that. DH would sometimes stop by on his own after work, and a few times, my own DM suggested I join him w/ our DD. But I didn't b/c we had our routines, and b/c I didn't think MIL would appreciate my just falling in on her like that (the kids, she wouldn't mind, LOL!).
I get your concern that the issue may one day extend to GC. But I don't think that will happen unless there are some tensions between you and DIL that you haven't told us about or aren't aware of. While I didn't bring the GC over to my ILs when DH dropped in on them, they saw each other often and enjoyed a lovely GP/GC relationship. If you stay pleasant and accept things as they are, I'm sure you'll get to have a nice relationship w/ any GC that you may have through DS and DIL.