The sister probably sees her brother more — so logically, she’d get attached to him.
Uh... no, outside of the incestbrained porn world (which sure, I'm a happy member of myself), that is not logical. Siblings are generally not attached that way, if anything because they see each other much more. (If you want two siblings to bang, raise them separately.)
In the real world, if someone tells you "a girl is attracted to either her brother or his girlfriend, guess which", you bet on the girlfriend.