Not being completely foreign to this type of situation, I can understand your predicament. If you do the "wrong thing", it can cause a lot of heart ache and potentially make already bad situations even worse. There is almost no ways to make everyone happy as contradicting expectations and conditions are the source of this conflict in the first place.

If it is a large group (and sound like it will be), you could potentially place which ever the one who is a target of most animosity on one extreme side and the rest on the other corner, having everyone else act as a buffer. You could also put the current couple on one side, the other woman on the other side, and kids in the middle. Those are just ideas but these too can be a source of further conflict.

I think, the best you could do is to talk to the party directly involved and find the least offensive way, telling them, if you can't find an agreement all of them will be out of the photos and the grand parents won't want that.

I'm sorry this type of things happen but they do - more often than some would believe. Sadly though, the images you create will have to be the reminder of these issues and there is no ways to just ignore it.