Recent Aggressive dog

Question
2 years ago my wife and I had a 7 year old male terrier mix, Harley about 45 lbs and got a female shep/rott mix, Reese at about 12 weeks. The pup was scared of anything unfamiliar and would run and hide if we had friends over, never showed any aggressive behavior. She warmed up to close family she got to see a lot, and got along great with Harley and other dogs. Reese also hated leaving the house, didn't like walks and never looks to comfortable riding in the car. Reese ened up about the same size as Harley, 45 lbs. Well this past October we had to have Harley put down, cancer in his stomach, we were heartbroken and Reese missed him as well. About a month later we got a 9 week old female German Shepherd, Crosby. Within the first week Reese showed some aggressive behavior towards Crosby, but no blood was shed and Crosby was ok. A little later we took both dogs on a trip to a relatives, Reese attacked their lab mix, a male about 7 years old, leaving a cut below his eye, and then attacked Crosby, she had a loose tooth and was bleeding. We were really upset and kept Reese on a leash inside the rest of the weekend. While on the leash she lunged at both dogs, but outside she was fine, they would play ball and there wasn't a problem. On the ride home Crosby fell asleep with her head on Reese, and they played when we got home and it seemed everything was fine. Until yesterday, Reese attacked Crosby when my wife was petting Reese and Crosby walked by, Crosby was ok and not scared of Reese. About 1/2 hour later, they were playing together and Reese attacked again, this time Crosby was bleeding in her mouth and it was outside. This was the first time Reese attacked while playing with Crosby. I don't know what to do, take them both to a trainer? Did we make a mistake in getting another female dog? Reese never showed aggressive behavior before we brought Crosby home. Even when she attacks, it's out of nowhere, no growling at all. We bought a soft muzzle that we have Reese in now, just so we don't have to keep them separate, but Crosby is scared of Reese. Any advice will be appreciated.

Answer
Two females is the combination most likely to have such problems, but she also attacked the male Lab Mix.  The behavior you describe may well come from a lack of early socialization.  Behaviorists call the 6-12 WEEK period the critical period.  Dogs may never be comfortable with things they aren't socialized to in that period.  The personality continues to develop until 3 years old.  She may have had genetic tendencies to aggression too.  

This is a tough problem, and well outside my experience.  Nobody knows everything.  I seldom suggest a professional trainer, but you may need to try one.  You might first try contacting some rescues for the Shepherds or Rotts.  These are dogs that lost their home, but were taken into a foster home to be retrained as necessary and placed in the right home for them.  You may find a rescue near you starting at http://www.akc.org/breeds/rescue.cfm  They have far more experience with poorly socialized dogs than I do, and may be willing to help you work with her.  If all else fails, they might provide a place for her.