My heart goes out to you, Mamma66! And to your GC who are caught in the middle of all this though, hopefully, they don't feel it right now.
I'm sorry you and DH (dear husband) didn't get to see them for 5 months. If it's any comfort, this is not unusual during a divorce, until a court order is worked out, as in your case.
I'm glad you get to see them every other weekend now. No doubt, both you and the kids get many joys from these visits. It was very kind and loving of you and DH, IMO (in my opinion) to open your home and your hearts to SS (stepson) and his children.
I'm sorry he doesn't seem to want to spend much time with them or do any of the childcare. I'm guessing he is very young, maybe early 20s? That's no excuse, of course, as he and XDIL chose to marry and have kids, and, no doubt, kids need love and care. However, it might help explain his lack of involvement.
I'm not sure why some posters are blasting XDIL, as well, at this point. She's taking care of the kids most of the time, if I understand correctly, and only wants SS to adhere to his responsibilities as per the court order. It's not her fault that SS tends to shift his responsibilities onto you and DH. IMO, that's immaturity on his part.
Hopefully, as the years pass, he will mature and begin to take on more responsibility for his kids. But, of course, that doesn't help you now.
Also, you don't say if he is working. Perhaps that's why he doesn't/can't pick up the kids from school on Wednesdays? If so, he should have brought that up when the court order was first written up, not just assume you and DH would take care of it. But I know, I know, that doesn't help you now either.
So I agree with what PPs (previous posters) have said - please see about getting the court order revised/Wednesdays taken out of the picture. Could you do it another day? Or will every other weekend have to suffice?
About her "insisting" that all his contact be "supervised" - Is this in the court order? If it's not, to my knowledge, you can just ignore it. If it is, can it be done, as a PP mentioned, by Social Services? This way, SS would be supervised and you and DH would get a break at the same time.
I know you're afraid that if XDIL doesn't get what she wants, she'll keep the kids away altogether. But, really, she too, needs to follow the court order. SS can always take her back to court if she doesn't.