Bitchy 12-year old cousin?

Arite, so lately, I've been at my aunts house and obviously, I have cousins. I have a 12-year old girl cousin that seems to be rude and bitchy at times, and I'm 15. the funny thing is, she's cute lol, but then she acts rude and then super nice at times. For example, when me and her are alone, she acts angel-like, nice, and cute. But then, when other people are around, like my sister, she throws an attitude at me. If we were the same age, I wouldn't mind this at all lol, but since I'm older than her, I can't really tolerate it. Plus, once, she was annoying the hell outta me by recording this video of me and she said she'll post it on Facebook. In the effort of trying to get her iTouch outta her hand, she raises her hand as if she was about to slap me. Even wierder, she smiles and everything when she's being like this. One more thing, my other cousin being a boy and 10 years-old is a bit better, but can act immature at times too. Being even older, he ignores me a LOT and then acts annoying sometimes, but not as much as the other. Lol for example, one time, all the cousins were sitting at the dining table chilling. There was a chair full of clothes, and just to make sure, I asked him, "Hey, can I sit here?" Jokingly, obviously, he says, grinding his teeth, with a seriously as hell face, "No, you can't sit here. Blah blah blah." Lol don't worry, I instantly knocked the boy down and got him quiet haha. Basically, these cousins are just not polite. So any tips? What the hell should I do? Lmao