Sibling kittens in heat

Question
I have two tabby mixed kittens that are turning 3 months soon. I was curious about when they are in their heat Heat stage and if they will try and breed with eachother??

Answer
Leslie,

I have seen females season as early as 4 1/2 months of age.  Usually, the males are not ready, but they can surprise you (unfortunately). I would have the little girl spayed at 4 months of age and the boy neutered shortly thereafter. There is much literature and success neutering/spaying this young in case your vet is unsure.

And, yes, they will breed with each other!!!!!!!!!!

Best regards... Norm.