P.G.
I certainly don't know all the facts, but a seizure? I'm darn glad she called 911. A high fever after a vaccination? Hey, better safe than sorry.
Your post seems to me to imply that your MIL was responsible for the childrens problems? Many kids do run high fevers after vaccinations. As for the seizure, I would imagine the doctors and parents will be getting to the bottom of the causes for that. If your MIL is responsible I am certain something will happen. As a parent, I would just be glad she got them to medical care instead of waiting for me to get there to do it.
She raised your husband(?) and his (siblings?). Now she's too dumb to take care of kids, especially yours? She's probably a lot more relaxed about it than you are because she does have experience. Is your Mom allowed to have only supervised visits as well?