OMG, I am so sorry about all you've been going through! Sending prayers for little DGS! And for you, DH, DSD, and the rest of your family.
Bless you and DH for being there for DSD and her child! I get that this isn't what you expected, but, IMO, you are heroes in their lives (DSS', too). So please take some comfort in that. I am confident that things will get better as time moves on and things fall more into place.
So sorry about the problems with DSD's mum on top of it all! Very kind of you to feel for her. But, IMO, DSD needs a restraining order against her. Maybe DSS, also. Unfortunate but true, I think.. But of course, they may not want that.
IMO, DSS' clingy behavior is a reaction to all this stress and havoc. I agree w/ you that he needs counseling to help him get through this. What does DH say?
Meanwhile, congratulations on your DGS, despite the difficulties! And please continue to reach out to us!