We had a similar problem, I have a 1 year old yorkie named Brady and in Jan. I got to adopt his dad, 3 year old Winnie.
For months everything was wonderful and perfect then all of a sudden Brady went after Winnie when a guest came over to our house. I don't know what set him off but for the next few days we had to separate them because with such a size difference I was SCARED TO DEATH that Brady could actually kill Winnie!
I was so stressed, not sleeping and going out of mind worrying about this and wandered into Petsmart in tears and spoke with a manager (after trying countless suggestions from family friends and vet) and was referred to a wonderful trainer. Brady and I are in private lessons once a week for 8 weeks to work on his behavior and after the first class we have had NO problems and I have never seen my baby boy so happy
The classes are not expensive at all, which shocked me!
I know it may be different for girls, but I would try working with a trainer. Since my Brady was the instigator he was the one I decided to enroll in class. My trainer uses C. Milan's techniques and he puts Brady on his side and holds him there until he calms down if he growls or is aggressive toward another dog. There is no yelling, just a quick aah aah noise and a roll over to get him to calm down. We have also learned sit, stay, leave it, come and are working on down! I don't know why that one is so hard! I use treats and praise as a reward and never strike my dogs or say their name in a harsh tone. All of this has worked wonders for me.
I also have read Tamar Geller's book, The Loved Dog, and I have to say that is a VERY helpful tool! I know that she does not believe in using the submissive roll over for training but in our case it was the only thing that Brady responded to. Other than that her methods are PERFECT for us!
I had to try everything in my power to get our problem solved and I would still try if I were you. I was a zombie for a week, determined NOT to have to rehome my sweet babies, how would I choose between them? I know how you are feeling and I want to let you know that there is hope out there for you and there is a training program that will work, you just have to be consistant with training and you will see a miracle! Pick up a copy of The Loved Dog the story will move you and C. Milan has books and dvd's to help with training also. If nothing works I hope you find a great home for your little girl on here!