I have a one year old cat that had never even seen another kitten...until one month ago.  We kept them from her for two weeks.  We paid more attention to her.  We couldn't pay attention to her when they were around, she wouldn't let us come near her!  It's been a month, and she's still hissing, although it has gone down a bit.  I think the problem is there are two of them, identical twins, and she feels overwhelmed.  All she knows is they're taking over.  When they aren't around, she's extremely affection.  It's like she's saying, "Make them go away, I'm your baby!"   I don't know what to do about this.