Or...is she is an abusive sibling?! There is always some sibling dynamics that can explain some behaviors.
But if it is hurtful, demeaning, degrading...meh, it's abusive.

I know a lot about this. Too much probably. I have had to deal with just plain meanness a lot and that isn't fun.

I can tell you one thing that has helped me more than anything. I Don't Engage.

Whatever you say can be used as a weapon later, so don't arm anyone with anything.
Change the subject or excuse yourself. You do not have to be anyone's punching bag. Ever.

Best to you,