I raised (am raising) litermates, but mine are 2 boys. They do play and it does sound rough, but they have never yelped or hurt each other. My boys are so completely bonded that my groomer tells me she needs to keep them in the same crate because they worry about each other when they are there. They look for each other in the house, you never find one with out the other. They share the same toys, eat out of the same bowl, share the same treat, sleep in the same bed, lay on the same lap (even though there are 4 of us there, they will pick 1 person (usually me) and they will both be there).
When I walk them they walk side by side always very close to each other. In the car they lay side by side on the seat. They protect each other when another dog or animal is around. Its so sweet to have 2 and they be litermates! If you have any questions or anything, feel free to contact me!
Fixing them in the near future when they are of age will help with the aggression also.