I know nothing about sororities, so don't quote me on this.
Try removing her completely and then putting her back in at a different time. I read it in the betta book I got from Petsmart and they said to do that when breeding betta fish, so maybe it will work for sorority tanks, too.
new members will have to "introduce" themselves and establish position. They will fight it out no matter what. But you could try putting her in a trap or float her for a few days before releasing. Both the new and old tenants will get used to each other and calm down a bit. This doesn't guaranty no fighting though. They might decide to establish rank later..... so keep an eye on them.
I find currents help reduce aggression. But don't forget to make certain areas current free, where they can rest and relax. LOL