After you split up the sisters, bring the injured girl to the vet but don't worry- even if it's a fracture, birds can recover very well from leg fractures. One of my budgies fell two feet off her perch and couldn't walk on her right leg. The vet did an x-ray and she had fractured her right femur, the largest bone in her body (she has bad osteoporosis). Since this large bone is mostly inside her pelvis he couldn't splint it- I just had to keep her comfortable and safe while it hopefully healed. I padded the floor of her cage and put all her perches less than six inches high. Today, three months later, she appears to have healed, at least enough to allow her to use her leg without pain. Sorry for the long solit up response. My budgie keeps pecking at the screen and messing up my message! Good luck and best wishes!