How is it possible to have some hot kids and some ugly kids?

I don't understand how that works, I thought family genes run in the entire family? So how can a guy who's really hot, have a sister who's not so hot? There's this girl at school who isn't that good looking, but her brother is. I always say to her, how can somebody as ugly as you be related to somebody as hot as your brother. What you think I'm mean for saying that to her? Well boo hoo, I don't care if I hurt her feelings, she deserves it for not having the hot genes like the rest of her family. How does that work though? Do the genes not get ped down the same to every kid? Wow I feel so bad for that girl, she got the ugly genes lol.