some female boxers when they reach adult try to get rid of the other dog.:( I know that is not what you want to hear. but I had 2 adult females that grew up together and played well till they reached adult. that is when gracie thought it was her job to get rid of mattie for no reason at all. it came to a point that they could not be in the same room. I would have to keep them apart all the time till one night when someone left a door open. that had to of been one of the worst nights in my life. both of my girls had to go to the vet but only mattie came home. gracie passed away that night. I often try to think if there was something else I could have done. but I know I did everything I could. all I can say is you need to keep them apart. maybe forever