M.L.
I'm very sorry. Perhaps she does feel like she is raising your child right now. If people ask, I would simply say to them, "I am on bedrest having a very difficult pregnancy and she is helping take care of him."
The majority of people will understand that, and if they don't then they aren't worth worrying over.
If she is living with your mother in law, then don't send your son over there anymore. Do whatever you can to keep your family intact.
Maybe your mother in law can come over some days to help, maybe you can hire a nanny for a short while to come over to your home and watch him for several hours a day, maybe you have some friends that can rotate watching him, or put him in a preschool for a while. Do whatever you can and rotate between people if you need too.
Just calmly remind your husband that her lies have no basis and you aren't sure what her motivation is, but you think she is endangering the closeness your family once had and really feel you all need to distance yourselves from her at this time. She is nitpicking minor things and blasting them way out of proportion. She is expecting perfect behavior from a child with a sickness. Remind your husband that you are having a difficult time in your pregnancy right now, and things will pass and return to normal soon, and that that he needs to remember who you really are.
It would be best if he just stayed out of it altogether, and not even entertain her by listening.