Brother and sister cats do mate with each other. Like all mammals, mating is a basic instinct for cats. ... The cat's body chemistry will tell it when to mate and when it's the right time for female cats to reproduce. So, cats will mate, even if they're from the same litter.

Can a female cat get pregnant by her brother?

Cats can become pregnant on their very first estrous cycle, increasing the chance that an accidental breeding may occur. Cats are indiscriminate, so a brother cat may breed with its sister, a father may breed with his daughter, and a son may breed with his mother.

Is it bad for a brother and sister cat to mate?

International Cat Care states that inbreeding increases the risk of the development of genetic defects. The organization acknowledges both pedigree and nonpedigree cats are vulnerable to inherited diseases, however they're more likely to occur in pedigree cats.