I don't think it's a case of not being your business as such, but I do think that it's more to do with him standing up to her and making sure that what he says happens. I'm glad you've blocked her and I think that's the most sensible thing to do. I'd also suggest that he puts posts so she can't see them - not unfriend him as he needs to see what she posts, but just so she can't see posts which she might have an issue with...
I assume it's all being documented? It's also worth documenting every time you take the kids to school, you look after them etc etc etc.... I'd also document every scratch, bruise etc that you notice - esp those done while you and J or his parents are looking after them....
