Hi I have a kitten who is now 9weeks old and had for 3 weeks and now have added another 6 week old to the family but worried as my 9 week old is pouncin on my 6 week old I'm not sure if they playing because she keeps making the hissin noise and a howl like noise what do I do as don't want to get rid of either x please help

Sounds perfectly normal. That's what kittens do when they play-fight.