Zellie, I'm glad you got to spend time with DS and GD! And thanks for letting us know!
I think it was very wise that you didn't discuss DS' marriage and just stuck to enjoying and talking about little GD. But obviously, something came up about DIL cuz DS explained that she was still not ready to see your family (or did he tell you this before the visit?) and asserted that he has to focus on her, is "grateful," and so forth. I hope this doesn't keep coming up in future visits and that you can accept it all as an ongoing situation till DS volunteers otherwise. IF you ask each time if DIL is coming or question why he doesn't try to arrange for them to see you as a family, and so forth (hopefully, you're not doing this, but IF...), then he's going to feel pressured and caught in the middle, etc. Not a good thing for him and it might just lead to his backing away.
I'm glad your complying with not contacting DIL. Disrespecting this request could only backfire on you. I must admit, I don't understand why that's so hard. As long as you're maintaining a relationship with DS and GD, even if not exactly as you'd like, why does it matter whether or not you get to speak with DIL?
IMO, Nina's post is great! The only thing I disagree with is that FIL needs to stop seeing DIL as "his baby girl," if that's what he's doing. She's an adult and needs to act and be respected as one.
Speldman, I'm so sorry about your DS' family's situation. But it's good that he's still a daily part of the GDs' lives. That doesn't always happen, as I'm sure you know.
I realize you would like to see him and the mom work things out and get back together, if only for the girls' sake. And so, DS' meeting a new woman worries you cuz it makes reuniting less likely. But as I know you know, the adults who are actually in the situation are going to have to figure this out for themselves, hopefully with the best interests of the kids in mind. Please remember, too, that living in a home with 2 parents who are unhappy with each other is not necessarily in their best interest.
Maybe the new relationship (if that's what it is) is just a "rebound" and maybe it's not. You can't see into his heart or his head, so no way you can know. If it's a rebound, it will be over soon enough. If not, well... deep breath... she may be your next DIL. So please don't say anything to him that he might repeat back to her/that might alienate her (chances are, you're wise enough to know this, yourself, of course).
Sigh... I agree it's good that you "can't get involved with what goes on..." I'm not even sure you could do that to the extent that you might think. Chances are, your only involvement would be to help out with the GDs, if needed. But being that close to the situation might just aggravate you all the more. And if you tried to give any advice to the adults, sorry to say, it would probably be met with resistance and might even cause you more upset. Even under the present circumstances, I suggest you just be supportive of DS and your GDs and that's all. Hard to bite your (general you) tongue, sometimes, I know. But, IMO, that's what you need to do (and probably have already). Best to you and yours!