I have a really rude aunt?

Okay, I need help. My aunt is really rude, she calls me things like stupid and says that I need to go to a hospital cause I'm insane (which is not true). And she does this whenever my mom is not around, like one time she called me insane and I started crying and my mom came in while I was crying and I told her that my aunt called me crazy and my aunt started denying it even though my grandma was in the room and saw her do that. My grandma told my mom the truth but my aunt keeps lying. And the problem is my mom tells me not to say anything to her if she does that and not to tell her to stop because she has cancer and my mom says we have to be nice to her. What should I do? I'm sick of my aunt doing this. I'm 13 and she calls me a crybaby.