hmmm... I inherited two dogs...a mother and a son. They were dreadful controlling little horrors. We tried, after a year of misery, to palm them off, together because they had always been. They were sent back as horrid. Then one day we were walking them and a person we knew said he would take one.
Since then they have both been charming little dogs. AS LONG AS THEY ARE ARE ALONE WITH AN OWNER and not in a shared household. Seriously. 90-95% less barking, fighting, walk aggression They were truly horrible and the guy who took the male was appalled at his behaviour when his Mom showed up). It would seem that they are just highly competitive, either by nature or nurture, and this shows in attacks on any other dogs in the family (our resident girl regularly beat up these guys who would start a fight.... only she was 5X bigger.).
Interestingly, both dogs are FAR worse, if we interact with them and our girl plays. She is a noisy player (sometimes upsets other owners and I have to reassure) and often this triggers REAL aggression from the two little guys. Competitive again. Triggered aggression. Again. Environmental aggression. Separate. No guilt. It just happens.
So. Maybe try the troubled girl in a house with no other dogs. Do not feel guilty about letting go. We had to.
Nick Peg n Benny (or is it Peg n Benny n Nick?)
(nTess, forever in my heart)