There's this girl who is trying to make me jealous because she is bestfriends with my only bestfriend, What do I do?
My bestfriend is my cousin (girl).
The moment you show that it bothers you. Then she will continue to do it.
She is doing it for your reaction. But don't worry! Most likely she just needs a friend. So be nice and she will not get the reaction she wants. Hopefully she will stop.
if you care about being friends with her be nice to her, if it doesn't matter to you just kind of ignore her.