play or aggression

We have two kittens who are brother and sister and at 11 weeks old they are a real handfull the problem is they love to chase each other (as fast as poss) but their play fighting seems to be getting out of control, fur is starting to fly at times and they have started hissing and growling when things at too rough, he seems to pick on her more and if she squalls as if he's hurt her he wont stop.

It does sound as if it is play that becomes too rough. Can you interrupt these play sessions before they get out of hand but tossing a soft toy into the mix or clapping your hands loudly?

Check their ears--if they're laid back, the cats are at least somewhat serious. Ears back means the claws are out and they're trying to hurt each other; they put their ears back so that the opponent's claws don't catch the edges of the ears during the fight.

Are your kittens neutered?

We wanted both the sister and bro. when we went to pick up our 3-4 mo. old cat years ago, but the owners wouldn't give him to us as they said he was getting too big and starting to make her life hard... and it can be quite a concern, so you might have to consider separate living quarters except when you're around. Discipling makes zero diff. to this kind of thing we've since learned more than a few times with our other male & female cats.

Personally I wouldn't spray them while they are playing around, it would be too easy to accidentally get them in the ears.
Shaking a can with pennies inside will get their attention.

If you haven't already, get them spayed/neutered it will really calm them down.

My cats are sisters and their play often turns too agressive. If I'm around, I make a loud clapping noise and shout "NO!" However, they, themselves, seem to give each other signals when the play gets too rough, e.g., growling, hissing, angry meowing, etc.

Once I heard my super-fluffy cat, Molly, growling and I did my "NO!" routine. Penelope, who knew she did something wrong, ran away from me and hid behind her cat tunnel. When I peered around the tunnel, I saw some very incriminating evidence in Penelope's mouth: a tuft of grey fur. She was trying to eat it before I saw her. (Molly was fine, by the way)

I would recommend the verbal commands as well, rather than wading in there with physicality. You want to make sure you're not favoring one or the other, in my opinion, as it might disrupt the pecking order they're trying to establish.

Kittens will play it they get to rough one or the other will settle it I don't intervine I'm not always home to do so and even when I am they settle it all by themselves. If they drawing blood I'd be more worried

They are only 11 weeks old so are too young to be speyed etc, we do shout and make loud noises when things seem to be getting out of hand, but it is very differcult to tell what we think is rough play, and them just playing rough and tumble.
Deb

At 11 weeks they're just playing. So long as no blood is drawn I'd just let them be. If they seem too rough try clapping your hands and distracting them - get them to chase a toy rather than wrestle with each other.

You can neuter cats at 11 weeks but it's not common to do it here in the UK that young and most vets will want you to wait. At that age they're not driven by hormones so I don't think that's a factor.