Cats avoid mating with their littermates. I remember an interesting experiment where the mother of a litter was made to small of a fruit and the males from the litter would then seek out females that smelled like the same fruit. They avoided their sisters if they were made to smell of the same fruit. Apparently the males seek a mate that smells similar to their mother but different enough to allow them to reject their siblings. A similar experiment was done with mice but the male mice would mate with their fruit scented sisters no problem. They would choose fruit scented non- siblings over non-fruit scented siblings however. If no fruit scented females were available they showed a preference for mating with non-siblings but would still mate with their sisters IIRC.
John Thornton