Define picking on her ... is she doing it in a playful way, tail up, non aggressive, or is she growling, ears back and in a snippy way?
Dobermans play rough, its just how they are. Yes, they tend to be same sex aggressive, but Ive been told its worse in males than females and that the females are more tolerant of the same sex. My two girls will play rough, and get rather loud at times, sounding almost as if they are fighting but they remain in a play state the whole time. Very rarely do they get snippy and argue. If they do they get separated, and after a time out they are allowed to be together again.
Now before I get jumped on I am not saying its not same sex aggression Im just saying it could be something else, Im not ruling anything out.