Oh dear
I'm afraid you have got the worst possible thing happening. Your youngest 2 are fighting for top female pack position. It's going to get NASTY unless they are spayed (or at least one) is asap. When girl dogs fight is is bad, they hold grudges and will fight to the death, sometimes the aggression can be life long. I'm not exaggerating, it really does get bad. You are going to have to separate the two youngsters from one another.
In severe cases 1 dog would have to be rehomed or they would have to be separated 24/7. It is quite possible that they will never get on again. Next time get a male
Your issue is definitely one of hormones and dominance, and I really feel for you
Edit: it could be even worse as they are siblings which add in a bit more competitiveness for them.