You have two options.
The first option is let the cats sort it out, as mentioned by @Gwendolyn and @trondhansen
Your second option is to intervene and personally I will always advice you to intervene, but that's just me.
If you do decide that you have to intervene then you have to invest a lot of time. So first ensure that you have ample spare time.
When kitten A, the stronger one, starts growling at kitten B, start petting kitten A. This will distract kitten A's attention towards you. Also don't forget to allocate some petting to kitten B. Otherwise, this might have negative mental impact on kitten B.
If kitten A starts relaxing, give her a treat, her favorite one. If not continue trying other methods to relax her, and then offer the treat. This is a type of positive reinforcement which your kitten understand as, if she doesn't growl at her sister a pleasant thing is going to happen.
Sometimes remove all the toys and put the two kittens in close vicinity and be alert so that no hostility breaks out. Don't play with them. To pass time read some book, listen to some music or do whatever is your favorite thing to do in past time. Don't let yourself to get bored. But let the cats get bored. Even if they nudge you don't play with them. Instead trying pitching up the kittens together. Use you hands to hold their body and simulate play fight among them.
If they don't show interest continue reading your book and don't play with the cats. Ultimately, the patience of one of the kittens will break, and the kitten will start play fighting with the other kitten. Continue this under your observation for a week.
Then give them back the toys. But ensure everyday, for at least 1 hour, the three of you must play when toys are not present.
Whenever, other than play time, hostility breaks out, take preventive actions. Like during meal time, if hostility breaks out, stop feeding them and pat them calm. Then start feeding only when they get calm. This is yet another positive reinforcement. They get the message that, when something as pleasant like eating food is going on, if they show hostility, they will get the food no more. But when they calm down the peasant experience will start again.
Do this type of positive reinforcement, whenever hostility breaks out. Like at sleep time, whose bed is which one, etc. Try to constantly improvise.
Hope this helps. Thank you.