https://www.seniorcatwellness.com/do-cats-mate-with-their-own-siblings/
Can Brother and Sister Cats Have Kittens?
Kittens can be sexually mature as early as 4 months old. Female kittens can conceive from around this age, while male kittens can impregnate a fertile female. This can be bad news if you have 2 cats from the same litter that haven’t been neutered.
If a female kitten becomes impregnated at 4 months old, there’s every chance that the cat could produce its own litter at only 6 months old. This is because a cat’s gestation period is just 63 days. Experts believe that this is far too young for a cat to be producing its first litter.
Cats are also indiscriminate creatures. This means a brother may mate with his sister because he doesn’t recognize her as family. More than anything, they are playmates.
To prevent sibling inbreeding, it’s best to get brother and sister cats neutered before reaching puberty. Or, if you’re looking for a couple of cats to become household pets, you could choose same-sex cats from the same litter. That way, you’re reducing any risk of sibling mating while still providing a playmate for one another.