Dear Sarah72:
Your aunt has bullied your mother for many years. Your aunt bullied you as well, for years. Your mother is now bullying you to visit your aunt, the bully that the two of you have shared for many years.
What you do next, I believe, is keep your assertion regarding your aunt, to not visit her. In addition to it, assert yourself further with your current bully. If your current bully, that is, your mother, keeps refusing to respect your assertion regarding your aunt, distance yourself farther from you mother, all the way to no contact at all, if that is what it takes.
Doing so will be the right thing for you to do for your own well-being. In addition, it will give your mother a good example for what one should do with bullies.
anita