Bless you and DH, Mama for being there for your GC! Same to all the other GPs here who are in similar situations. You are the heroes in their lives.
I'm so sorry their mum isn't fully responsible even now after her CPS scare. TG you and DH provide the kids w/ as much stability as you do.
I feel especially for the oldest one. Clearly, she is being made to play a parental role in many respects. Is Social Care aware of this? As close as she is to you, I understand her wanting to live w/ her other GM since she, basically, raised her, according to your comments. Does she get to see/talk to that GM very much these days?
It seems as if SS (stepson) is SC's main concern. If they only want him to have supervised contact, for whatever reason, then it's probably not a good idea for him to have the kids alone too much when that restriction is first lifted. I think he would do better to ease into parenting gradually. But from what you tell us (sigh), he doesn't even seem interested in that, anyway.
IMO, it's beautiful that you have tried to influence him to be a better parent. But, it's not your job, really, and at this point, I think you would do better to just accept who he is. In time, you may have to help the GC come to terms w/ that.
My heart aches for you and DH - and your GC. And I totally hear your comments about feeling "shattered." I'm not sure, though, if that's b/c you are so disappointed in the parents, especially SS or b/c you are finding it increasingly difficult to take care of the kids for whole weekends and don't see an end in sight. Or... Can you clarify?
I agree w/ the poster who pointed out that these dynamics will change as the kids get older and more independent, etc. There is a light at the end of this tunnel. You will get through this and probably have many fond memories as a result. Still, I know you're stressed and hope you can arrange for some help, after all. Hugs!