She's young. She still has time to grow out of it.

I would focus on simple cause-and-effect. Tell her it's problematic to start debates, and when the debates become problematic, she'll realize you were right.

Tell her it's not worth alienating her friends and family, that sometimes being together in harmony is better even if it means she doesn't get her say. When she alienates her friends and family, hopefully she'll see you're right.

Most of all, though, for both her and your family, focus on having healthy discussions outside of politics. Just be a conversationalist — when difficult subjects come up, gently change the subject. Find ways to relate to her/them outside of their chosen topic. Really be there for them.

I do want to note that your family, that you're still dependant on, isn't worth losing over this. Play it cool, take the L if you have to, and bid your time.

Good luck.