I'm 14 and a 30 year old is gossiping about me?!?

[Kayla]

There's many things you can do:
Confront her! Go and tell her you read the text and told her you're not. It's very stupid of her to think you're homosexual for liking bands and taking pictures of yourself.
If you want, also block her from your Facebook. If all she's doing is spreading rumors and stalking your page, she doesn't deserve to even look at your profile username!
Tell your parents about it, and see if they can help with that.
Be the adult in this situation: do the same thing - confront her about it and tell her how disappointed you are and for her to spread rumors about you and to assume something like that, with her being YOUR cousin, and probably seeing her and interacting with her a lot over the years, or however long you knew each other, was very childish and/or absurd of her to do such. If she tries to protest, stand your ground and tell her no matter what she tries to do to change your mind you'll still think what she said was wrong.
If she apologizes, simply forgive her, it wasn't much of a big deal, but still, watch out for her, you don't know if she's gonna do it again, so watch.
If she tries that "I was just kidding" or that "Come on, I was just joking with [Insert name of girl she was talking to with HERE]!" OR that "I was just pranking her to see her reaction" crap, don't fall in for it, but if you think she's being honest, just give a "sure" or a "whatever" or tell her that on your part, that wasn't really nice of her.
Hope I helped!
Regards,
Kayla.