What does she mean by "he turns over in his grave"?

Native speaker’s answer
Rebecca
The phrase "to turn over in one's grave" implies that the deceased would be uncomfortable, upset, or offended by the actions of those living. So here, she's saying that having Micheal live with her and her family would upset her dad so much, that he's probably "turning over in his grave". Ex: My grandpa would turn over in his grave if he knew I quit being a lawyer. Ex: I can't believe we're moving out of our family home. Great-grandma is probably turning over in her grave.