Depending on what you want to do. What's your main goal? Getting them out of your bed, them staying in their beds, them just staying out of your room, ect.... You can:
1)escort them back to their beds EVERY time they get up and eventaully the will get the clue that they are not getting into your bed.
2)tell them that they are allowed to come in to you, but not allowed in your bed. If they must be near you they are to sleep on the floor.
3)turn the lock backwards so they can't get out of their room at all. (You'll end up a couple times with some crying and a couple times with a child sleeping on the floor in front of the door!) But again they'll figure out that sleeping in the beds is much more comfortable!
4)baby gate their doorway. Same effects as the above option, just some kids deal better if they can see what is (or isn't) going on on the other side of the doorway.
5)Tell them they can't sleep with you, but feel free to climb in bed with the other sister!! That will really get them!!
IF they do share a room I could see how some of these might not work. And if that's the case and I were you then I would just go with 1,2, and/or maybe 5. No matter what you decide to do the key is CONSISTENCY!! But that's just me and that's my opinion.
You do what works for your family! That's what parenting is all about, doing what you think is best for yours! If not anything else they will eventually decide that sleeping with you isn't "cool" anymore!! Lol