I'm happy that you got another kitten for you cat. It'll do him good. And you'll see, there's so much more love and fun when there are 2 of them.

I once had 2 kittens, sibblings. There's nothing to worry about as long as the female isn't getting in season. You'll notice it: she'll roll over the floor, and all the other signs of it.

Here's what I got from Emazing:

Early Spay and Neuter
How old should your cat be before it is spayed or neutered? Many humane societies have been doing spaying and neutering in cats as young as eight weeks for some time now. Many veterinarians like to wait until the animal is six months old to alter them. It is not necessary for a female to go through a heat cycle before she is spayed. The earlier you have a male neutered, the less likely it is that he will start spraying behavior

Why don't you talk it over with your vet?