Doctors and nurses will both tell you and your sister the importance of taking pain meds early on. The sooner your sister gets up and moving, the faster she'll recover. Pain pills will help her work harder and rest better.
Narcotics like Oxy Contin or Percocet are often used for moderate to severe pain after major surgery. Physical dependency on drugs of this type occurs with long-term use. Most patients after hip replacement only need this kind of pain control in the first week to 10 days. The idea is to get control of the pain before it controls the patient.
Encourage your sister to take the drugs as they were intended to be used: on a short-term basis to control the pain and allow early movement. Hopefully she will see the common sense of this approach. Ask the nurse and doctor to keep explaining it to her. With a team approach she may be more willing to yield on this issue.