The question we should be asking here is whether it is okay to rate a person based on their looks (it isn't), whether a person's looks are relevant to this situation (they aren't), and whether this person's sexual life is anything to do with you (it isn't).
She has the right to sleep with whomever she wants (as long as it's legal and consensual). If the people she sleeps with are related, that is still her decision and that is still okay. You have no reason or right to judge her based on who she decides to sleep with.
My advice to you would be to develop some maturity and realise that other people's sex lives are nothing to do with you and that when you 'fancy' someone you need to 'fancy' them for THEM, not their looks and who they've slept with before.