My cousin is going to tell my ex crush I liked him?

A few years ago I had a maaad crazy crush on this guy
I'm pretty embarrassed by how I acted for that year, literally crazy, and most of my thoughts were sent to my cousin who's a few years younger than me
I know that was a bad idea, but I wanted to be able to talk about him to anyone who didn't know him
I don't like him anymore, and haven't for a year or two

Now, she goes to the same school as me
She's in Year 7 while me and the guy are in Year 11
I saw a message from my cousin to my sister saying that she had seen the guy and was going to talk to him about me
Which, not only is EXTREMELY embarrassing, but I hadn't told my best friend and if it gets out, she'll be really upset/offended

What should I do??
I doubt she'd listen if I told her not to :/

****, I'm not even joking this has happened to me. Well, ish. When I was young; I found crushes embarrassing... well, because they are! I was too embarrassed to tell anyone, apart from my two younger cousins who I thought would keep quiet, because sometimes, you just have to let it out. After a while, though, they started to threaten telling my parents (they were young and it was more a jokey way) but I really really didn't want them to find out... so my (ten-ish?) year old self would have to practically CRY to make them stop. Honestly. Even though they've probably forgotten about it and it's been years and years, there's that little 'alarm' inside me which makes me think 'oh no' when we see them... yep. It's ridiculous.

Anyway, time to help YOU. Are you good friends with your crush ? Say something like, "oh by the way... my younger cousin has a massive crush on you, and yesterday i (insert random thing that'll annoy her.. maybe eat her chocolate) and she'll try and get revenge by telling you or (insert random person name), well, that's at least what she told me, don't believe it." Or something along those lines. Something which makes it seem like your sister is lying etc etc. Therefore, it's settled with - completely. Once it's said, there's nothing more anyone can do, and it won't really be that juicy anyway if you knew it was coming. Hopefully I make sense... trust me, I know how it is. It's like you'd practically do anything to go back.

Aw my cousins are exactly the same, it sucks
I'm not good friends with him at all though, actually I'm sure he finds me really weird because of how I acted when I liked him :/ Like, it'd be awkward that my cousin even knew who he was, because that implies I've mentioned him