It is a long story, read about it in Pet Group.
Anyway, We had a 10 yo male neutered (Dog 1), got a female puppy, spayed when eligible (Dog 2). They got along well, the male was a good playmate for a while but started slowing down. So when Dog 3 was 3yo, we got her a little sister (Dog 3). She hated her little sister. Our vet even spayed Dog 3 early to try to help the situation. Many vet trips for dogs, and emergency trips for husband. He did not learn from first bite, second bite got his finger. Never use your hands to break up a dog fight, DUH!
We went to behavior vet and he said Dog 2 was trying to become alpha because Dog 1 was old and not really a threat, he minded his own business and left the 2 girls to do their thing.
Well when Dog 3 started getting a little bigger, she quit submitting to Dog 2 and the Behavior vet told us we were to blame.
Whenever Dog 2 would try to exhibit dominance over Dog 3, we would step in and scold Dog 2. Dog 3 started thinking we were behind her and she did not have to submit anymore. Vet told us we had to work on changing how we treated the girls. Dog 2 was to be fed first, get treats first etc. Dog 3 was to be crated or scolded when fights started to show her she was not alpha and should learn to submit.
Anyway, husband moved out, took Dog 2 (the one that bit his finger off). Dog 1, my beloved had to put down in Dec.

So now it is just me and Dog 3. She is queen of the house and loves it.
Unfortunately we did not get to do the behavioral training before husband left, so I can't tell you if it will work. But the idea made sense. The Behavior Vet said, ultimately the humans are the alpha then you can help set the hierarchy of the rest of the pack by the way you treat them.
BTW, Husband's finger was sewed back on.