Can Sibling Cats Mate?

Would brother and sister cats mate with each other?

Unlike us humans, cats do not care about family ties when it comes to mating. A male cat would mate to his sister cat or even to his mother cat especially when they are the only available female cats around. This is the same for female cats. If the only available male cat is their brother or son, the female cat would mate with him if she is already in heat.

Would their litters develop deformities?

According to Cat Pregnancy Report, there's a higher chance for the kittens coming from sibling cats to develop birth defects and diseases. Nevertheless, that is not always the case. Most kittens are also born healthy with no deformities.

Also, based on my own experience, my cats do not have any deformities at all despite that their parent cats are brother and sister.