hey laura. I'm a physical therapist and I've treated many post op knee surgery clients. I'm surprised your sister didn't have any PT after her surgery especially if her leg was inclined to atrophy(depends on her condition and activity level). Anyway she should have a program developed that includes straight leg raises in all four planes with ankle weights when appropriate,bicycle and closed chain exercises like squats/leg press. That would be a good start. One more thing ...this all depends on her current level of knee/lower leg condition and if she is experiencing any pain,anywhere in her leg. Good luck,Dorothy.