Unpredictable mother in law ?

It's important for parents to spend time with their children. I think your mother-in-law is afraid of losing her position in your household. Your husband may have observed something that you didn't. Discuss that with him. In the meantime, just watch and see. Your mother-in-law's behavior can't be good for your children. Watch, wait, observe. She may be saying things about you when you're not there.