Okay, I have two female kittens they are sisters around the age of 3 months. We got the kittens when they where 6weeks old. But there is a problem, one of them sucks the others breast?

LOL I had a kitten suck on my oldest dogs boob. And he was a boy dog. That's not a problem. At all.

Okay. Now this made me laugh!!
You might of got the kitten too young when they weren't weened and ready to be taken away from their mother. They will grow out of it though, don't worry.

There is nothing wrong with that, the on just misses his mom, and or wasnt weened off the milk early anof causing her to be a sucky baby, I have 2 male cats an the one will randomly start suckking his fur when he is tired, it's a normal thing for kittens to do things like this because they miss there mother thats all.

My 1 and a half yr old cat does that to my 5 year old cat........It shouldnt be a problem, if the other cat doesnt like she will let her know

These kittens were separated from their mother too young. Kittens should be with their mother until at least 8 weeks. The suckling is not a problem, but it might be a life long behavior. It's good that you have 2 of the siblings so that they can comfort each other.