You can see that the girl wants to, but as I understand it - her mother did not often let her go to parties. Slim and pretty, she wants to have sex and get off like a grown-up! And the guy's got a big dick, and he handles it like a man. The little sweetheart's got a hard-on. But she's riding his dick with gusto. I'm telling you, she's got potential. At the end of it, she's getting fucked in the ass! She's just twitching in ecstasy.
She's really got it in for her brother. There's no point in arguing with her, telling her off. So at least he played out all his anger during sex by pounding her between her big buns.