Oh, sorry, didn't see your latest post! Please ignore the above!
So shocked at the authorities' casual attitude to what was going on with OGS! And so deeply, deeply sorry...
About the other kids - You must be besides yourself! I'm not sure what I would do in your shoes! Hopefully, the websites Ginny gave you will help.
It's clear DD feels overwhelmed by the childcare (on top of her fulltime job) and perhaps SIL does, as well. I know, I know, other parents are in the same boat and they chose to have several kids, etc. But apparently they are having trouble handling it all (sigh).
I'm glad you recognize that the fact that DD blocked you on FB suggests "she needs some time to cool off." I trust you realize that means you shouldn't call her or anything, till she calls you. I know you'll be worried about the kids, but you are, anyway, and trying to contact her won't change anything. Chances are, she'll be contacting you again soon for babysitting, if not this weekend (maybe she already has?), then next.
Once things are back to "normal," for a while, is it possible to offer to come over and watch the kids for a few hours each day, while she and SIL sleep/unwind after work? I know that sounds crazy on top of having them for whole weekends, but perhaps you and the other GPs could take turns doing this? Or offer to all chip in and pay for a nanny/sitter to watch them? These offers would have to be made in a supportive way ("You two have so much on your plate and we want to help..."). And I know the parents may well decline. But it may be worth a shot.
Since you just had a blowup with DD, I don't think you should be the one to present this idea to her if all the GPs agree to it. IMO, one of the others should float the suggestion, either to her or SIL or both. I know this isn't how it should be - they should be there more for their kids and so forth. But they're not, so I'm trying to figure out what might help.
But first, forgive me, I feel compelled to ask, do you know for sure that DD and SIL leave the kids on their own while they relax upstairs? Or that the toddlers sometimes turn on the stove? And so forth? Are you certain you're not getting a distorted version of what goes on now from the kids or whoever? After the tragic loss of OGS, I don't blame you and the other GPs for believing the worst, but before you do anything, IMO, you need to be clear in your mind that the situation is as bad as you've been led to think.
(((Hugs!)))