First, LP, my deepest condolences on the loss of your beautiful DD. What a tragic, sorrowful event, for you, your GC, and yes, your SIL, as well.
I agree w/ inkycog that there are many threads to this story. In fact, I admit I'm somewhat confused. Although the situation regarding the original nanny was disturbing, it seems that she was finally fired. So, like Hetty, I can't help but feel that the children's lives are better now. I know you don't like the fact that SIL's new GF (girlfriend) lives w/ them. But if the kids get along w/ her, and she's basically good to them, I don't see a reason to worry. Also, I get that the impending move concerns you. but maybe it will be good for the GC to get away from the memories in their home and start a new life. Sure they'll miss their friends, etc., but perhaps they can keep in touch w/ them via Skype, etc. Kids today don't lose contact w/ old friends when they move as easily as they did years ago.
No doubt, you'll miss them and they'll miss you. But I'm sure you'll get to see them and even have them over when SIL and GF go away on those "childfree" holidays. And I trust you can stay in touch w/ them, just as I said about their friends, via Skype, FaceTime, snail mail, etc. If you're cautious not to pressure SIL about this, I'm sure, in time, he'll allow it. The move will be hard for you, at first, but I'm sure you'll adjust in time. Hugs!