If you can isolate her for a couple of weeks she will have to res-establish herself in the pecking order and there is a good chance her behavior will be corrected. If you can't figure out a way to do that she has to go.
I currently have 1- 10 week old RIR, 1-7 week old Buff Orrington and 2 - 5 week old Black Aussies. The RIR and Buff have been together for about two weeks with no issues. Adding the two younger Black Aussies resulted in about one day of moderate pecking by the RIR and the Buff. After one...