Hi and welcome to YT!!
I am sorry you are having the problems with agression but it is true that the females need to be separated when pregnant, in heat or after whelping as they become extremely protective. The bigger female may also be jealous, or if she is the alpha, she may sense the new resistance to submission by the momma. Some females will even eat another females pups.
I am aware of a recent situation where one female actually killed another. So sad. You are right to be concerned as this apparently happened when the owner wasn't home.
Congratulations on your litter and I don't want to throw you to the wolves so to speak, but I would love to hear your experience with breeding your little girl. There are mixed views on this board about the appropriate size for breeding and I wondered if you had her evaluated (pelvic capacity, etc)prior to mating? Did she free whelp? Were there any complications and how big was the male? I am guessing it was her first litter since she is 18 months old. If you would rather answer me privately, please feel free to PM (private message) me instead of posting the answer here.
Again, Congratulations and welcome!!
