My cat, Trevor, reacts strangely to some visitors too. When someone new comes into our home he rubs up against them and smells their shoes. Sometimes he will give a muffled "growl." We have tried to think of a link between the people who get the "growl" and those who do not. The only thing these people have in common is that they have dogs at home. This is sort of strange, too, because Trevor used to live with 2 dogs and got along quite well with them. (They have both since passed away). It may not be the "dog factor" but I would agree that it is possibly a smell that triggers the reaction. Trevor is also not a kitten. He is a big 22 lb. tabby who is not reacting because he feels threatened. He is normally a very laid back cat! There was one incident on Thanksgiving a few years back where he did hiss at my 14 year old niece but I think it was because the house was full of people and he felt a little stressed. Up til then we had never even heard him hiss at all (except the day we introduced him to our new kitty, Andy). Now they are great buddies. I would say give it some time but, by all means, don't leave your kitten in the room with your niece until he outgrows this behavior.