Why Do I Get Hated On?

So my cousin, (let's call her S) I was basically raised with her, and I always knew she hated me because she's told me before when we were little (were about a year apart, her being younger). But I always ignored, and thought it wouldn't affect me. Well, 1 year ago she started going to the same school as me (I've been in this school since I was 4, turning 14 now), and she keeps starting rumours about me. In fact, this her FOURTH time spreading a rumour. I've talked to her about the last three times, but now I'm done and sick of it. I've had my parents talk to her parents, my parents talk to her, I've talked to her, my friends have talked to her and she never stops!! Everyone in her class basically hates me now, and some friends have turned against me because of the things she's said. I want to talk to her about it tomorrow, but I know nothings going to change. I just don't know what to do anymore, she claims I talk about her but I swear on my life I don't!! And when I do, it's about fun times we've had together!! She's the one always talking about me, saying I'm rude and disrespectful (when she yells at her parents mostly her dad 24/7) and she says I'm an idiot (despite being first on honors for 3 years in a row), she also says I say she's jealous of me?? I only thought that because she's TOLD me HERSELF that's how she felt, and she claims she never told me that. I need advice. What can I do to get her to at least dial down?