
I had the same problem with Chase and Lois, Lois was nearly 2 when she joined my clan and Chase was 6 months younger and already put herself in the top spot when she arrived 10 months earlier, Rhuari being besotted with her just went with the flow, the girls got along fine, not even a growl when one stole a toy off the other, then Lois had pups[Chase had had 3 phantoms by then] nothing different till the second season after the pups when there was an almighty fight, they were hanging on to what ever bit they had grabbed and not letting go, tried making loud noises but there was no let up, grabbed them by the scruff and shook them, still not letting go, so decided to prize their jaws apart, guess who ended up bleeding badly when in their attempts to get a grip my thumb got in the way.
I eventually got them apart,blood everywhere which turned out to be mine and separate them to 2 crates and patched up my wounded thumb, a tooth had gone right through the nail, went back and examined the girls and other than wet hair there was no obvious injury[found a couple of tiny scabs the following week when grooming them], spoke to their breeder who said it was probably Lois who had started it, that having pups had raised her status and she wanted to take over Chase's crown but she wasn't going to give it up, after the seasons were over I reintroduced them under supervision and hoped that it was just a one off, it wasn't and happened again a few months later,like previously I didn't see a trigger and it was me who got injured so from then on unless they were on the sofa either side of me one was always crated, not in a separate room and usually paired up with one of the lads at night when both were crated.
Lois's daughter Lexis was never on the receiving end of any of this aggression thank goodness, she was treated like the lads. My long term plans were to rehome the girls together when they retired from breeding but that idea was scuppered, I don't know if spaying would have worked as Lois died suddenly during a brief illness, I had arranged to rehome her and her daughter after she had another litter for me so instead Chase went with Lexis, the latter returned after 4 days because she wouldn't settle.
My fear is that Lexis and my new girl Maizie get into the same fights when the latter eventually has pups, at the moment there doesn't appear to be a head of the clan but that could all change on motherhood, only time will tell.
I wouldn't wish this bitch fighting thing on my worst enemy, you are on pins 24/7 that things will flare up again and that serious injury would result and not just to me this time.
I hope things settle for the OP but I wouldn't bank on it.