You have siblings, who are both females and spayed. Just about the worst combination you can have, and the fierce fighting for domination is going to get worse.
I don't know how far they will take it.
In a case like this, one might actually kill the other or both may be gravely or fatally wounded.
You are going to have to keep them separated, or find a new home for one of them.
Big dogs fighting fiercely is a very dangerous atmosphere when you have kids around.
It also sounds like you are pretty lax with the dogs. I notice you say things like "She lets us"...."She allows us." YOU are supposed to be the boss, and if you aren't, then for sure the dogs are happy to take over that position.
If you rehome one of them make sure you take the other to obedience school so you can learn how to train and get control of a dog.
If you rehome Biscuit, you must find an experienced adult home.