Do your pets all get along?

My two cats are from the same litter, they have never lived life apart. They like each other and my female cat gets very upset when she is alone, she just doesn't know how to cope without her Brother! The boy cat though is fairly dominant with her, and he plays very roughly with her. I am sure it is just cats showing their dominance, and playing and things like that but it is still pretty bad when he pins her down and she begins hissing at him.