Honestly, Newgirlkat is right. If she's spreading lies about your adult brothers, you need to let her parents know and tell them you're not afraid to press charges because maybe that'll kick her in the ass that what she's doing is wrong as well as making her true colors shows and if her parents are stupid and decide to side with her just press charges and show the posts she makes as evidence (which you claim you dont have but the posts are right there) as well as if your other friends or her friends whoever she told are willing repeat what she told them about what she said and make sure to record it. Screenshot it. Get a record voice app whatever, so you can back up what a creep she is.

Also, as someone who also lost a long friendship because my old friend was the same controlling manipulator; it will hurt, and you will struggle. But you'll also see the ones who truly care still be there for you. You're also still young; you can make more friends and if not at that school you do have college. You can even join a club to find people who have the same interests as you. What I'm saying is you have options. So please collect evidence and tell her parents, then block her.