Ideally, kittens should stay with Mama Cat until they are about 12 weeks old. That's when Mama teaches kittens social behaviour as well as other survival kitty skills. Don't feel bad if this is the case. Popular thought is that 8 weeks is plenty of time for Mama to teach her kittens everything.
In any case, Mama may not have taught your older kitten how rough is too rough.
If it is just inappropriate play, you can try startling older kitten out of the behaviour with a noisy shaker. Take an empty pop can and put a few pennies in it and tape off the top. When you see/hear that it is getting too rough, shake the can. Older kitten will start making the association that when she gets too rough, there is a bad noise that happens. Consistency is the key when training cats that any behaviour is inappropriate, so unless you can be with them all day I would separate them when you are gone and have supervised time together.
I'm sure others with more kitten knowledge than I will respond with suggestions shortly, too.