I haven't made it through all the replies, but I do have some thoughts.
First of all, I agree with the other posters that you should get out of Dodge. I would not live anywhere near these people. If it means that your mom visits, so be it. Move to a place where she is more likely to visit. Yep, Florida sounds good.
Document everything that has happened, been said, etc, with dates.
Make your mom the guardian in case something happens to you and your DH.
I don't know if there is a sliver of truth in anything that your SIL has said, but if so, there are wonderful parenting classes. I
not saying that there is any truth to it, just that parenting is the hardest job in the world and if there are things to work on, there's nothing wrong with a class. It's not admitting failure, just working on some skills. Not that I'd tell your SIL and MIL about a class...