mating among siblings

Question
Do brother and sister cats mate with each other.  How early do cats start heat?

Answer
Brenda,

Yes, brothers and sisters will mate.  In some more precocious breeds, I have seen females begin to season as early as 4 months of age.  Similarly, males may be ready as young as 5 months of age.  If bred at 4 months, the female could have kittens at about 6 months which is altogether too, too young to be having kittens!

Neutering or spaying at 4 months is a perfectly acceptable practice amongst more and more veterinarians.

Best regards... Norm.