Though it has occasionally hurt my feelings to be on the receiving end of it, I have learned a lot from watching other people enforce their boundaries. It has been quite inspiring actually.

You can be a role model and very positive force in your niece's life by giving her the experience of what it looks like when someone lovingly and firmly insists on being respected. By saying "Alice, honey, talk about eating disorders is off limits with me until I tell you otherwise" you are teaching Alice that she is allowed to protect herself and declare some areas of herself off limits to others. When she pushes you, you can redirect once or twice ("Alice where did you get that cool T shirt?" Or "Alice how was the car ride getting here?"). Then leave if she persists ("ok that's it for today, I'm going to have to run now.") This teaches Alice that being disrespectful has consequences which, frankly, it sounds like she needs to learn yesterday. Also you will enjoy her more (and she will like being enjoyed) if you are not terrified of her.

This is not mean. You are being respectful to everyone involved, you are protecting you (which it sounds like no one else does) and you are being a wonderful aunt. You are not doing Alice any favours whatsoever by letting her walk all over you, trigger you or terrorize you. It is not a kindness to Alice to allow her to behave in an obnoxious or abusive way.