A couple of questions

When do cats usually start to move out of the kitten-dom? I mean do they start to settle down into cat personalities? Mine are 6 months and they are starting to LOOK like cats, but still act like kittens. Seems like a year is a turning point but can't remember.

Second, my boy is definetly the alpha cat to the girl (they are litter mates). Now, I have been thinking of separating them more and more-like put him in another room for a few hours a day, cause he just attacks her so much I think she her real personality isn't allowed to develop. I mean he just attacks her all the time now.
I know it's all in normal play, but I want see if she would be more comfortable with some time on her own. Or would that be mean, to keep them apart for awhile?

Are they spayed and neuter yet ? If not that's where the agression may come from by the male cat . He got the right age now to get neuter and the girl spayed too . Yes I believe around 12 month they start to settle down .

Yes, they are spayed and neutered.

He is really aggressive towards her and just in general right now. But REALLY towards her. I really want to see how she would do on her own for a bit.

You can try to get the Feli-way plug-in , that helps bretty good and has helped me a lot . You can get it on line or at Pets -Mart . On line it is a little cheaper as far as I know . I wanted mine quick so I got it from pets mart .

Thanks, yeah I just ordered some flower essence. I had somehow thought I was past that (used the transition one for when they were new here) but I think it's in order. hopefully it will help!

If littermates stay together, and they're spayed before they start to enter kitty puberty, I find they stay kittenish for a long time! 6 of our cats are littermates, and they still act like kittens a good deal of the time at 2 1/2 years old! They groom one another, wrestle, chase each other through the house, etc. They've never had any reason to grow up!

That an interesting point. I see that here. These guys have it so good, I mean they have the run of the place when I am at work, they have their original siblings to play with so I can see them never growing up! Why should they! I just hope they do stop fighting so much, he is out of control sometimes.