They just won't. They're really close. Teammates won't even fight each other (DC vs Cain; Askren vs Woodley). Why would you think biological sisters would fight each other? Although I'm sure they aren't opposed to making money, they seem to be in it for love of the sport more than fame and fortune.
EDIT: I was being lazy assuming everyone knew this already.
“I don’t think so,” Antonina said. “Of course, we’ll never fight as sisters. Valentina now is No. 1 contender at flyweight. I’m sure in a few months she will be the champion of this division. If I win this fight in Contender Series, I will be just entering UFC. There will be a hard, long way before you become No. 3, No. 5, No. 1. it’ll be a long way and hard way, so we’ll see.”