hi we have 2 kittens but from different litters. the female is 8 weeks & very playful and friendly, the male is 10 weeks & keeps hissing and growling at us and the other kitten, we keep petting & talking to him in short spaces but he doesnt seem to want to mix. we have had them for one week now.
As with humans, cats have different personalities and react differently to situations. Where as some cats are quite happy to get on with new cats others feel threatened and scared. This uncertainty is often displayed as aggression and that what it sounds like your little boy kitten is doing.
I don't know how you introduced your cats so i will just give you my advice regarding introducing new cats.
New pets should be separated for at least a week to get used to their new surrounding and also to become familar with all the scents. Smell as you know is a major communication method for cats and is very important for them to feel secure. While they are separated, try to introduce each cats scent to the other via bringing someting into their rooms which has the others scent on. Also you can rub a towel over one of the cats and then rub the towel around the table legs and doors etc in the other cats room. During there time apart they will become familar with the other scent so that when they do meet for the first time they will be famiar with their scent.
Introduce your cats slowly to each other a little at a time. Let them spit etc at each other if they want to, it's just their way of asserting their authority. generally cats will sort out there own socal order.
Your boy cat will become more confident as time goes by and will become familar with your other cat. You must realise though that you can't force a cat to like another. It may just be that they never become true friends but just tolerate each other. This is quite normal.
You are doing all the right things by talking and patting him regulary, this will boost his confidence and i hope that in time he learns to love his little sister too.
Kate and her partner co-write a site full of further cat care and cat health topics. As well as being a feline friendly community site for happy cat and a happy owner.