Welcome Jaffa! I'm sorry about what DD is going through but glad she is feeling better. I'm sorry about how it has affected you, as well, but hope you'll be feeling better, in time, also.
I'm another one who thinks you should focus on being supportive of her and not get in the middle between her and SIL or his family. What they said to her was very cruel and unfair, but she's an adult and can, no doubt, answer (or not if she prefers) for herself. If you step in she may see that as if you're treating her like a child. Not to mention it could backfire on you if she and SIL ever get back together (and please don't think that's impossible).
But as long as they're not together, I see no need for her and her XILs to remain friendly. It might be nice, but she doesn't really need them in her life, surely. Eventually, when she finds someone new, she may not even want them in her life that much, just in the kids' lives.
I agree with those who said the main thing is that they are good to the children. In fact, I don't see any harm in their posting pix of their activities with the kids on FB. These events occur, if I understand correctly during SIL's weekends with them? And DD is glad to have the free time? Then there's no need for them to include her or to be afraid to post their pix. Unless they're badmouthing her to or in front of the kids - and I hope not - I don't see any harm in their enjoying these times with the GC and posting about it. As a PP said, if you or DD find it painful to see, just unfollow them.
I hope DD's situation continues to improve and that she surrounds herself with people who care about her, not those like her XILs who can give her nothing but negative energy. Wishing you, DD, and your GC all the best moving forward!