My sister is having a mastectomy, what are the right things to say to her?

You are her rock. Iif the medical decision has already been made to preform a mastectomy - meaning you were there for her to aid in listening and asking questions and helping her make an informed decision under the guidance of the doctor---then your job now is one of support, comfort and building for the the future - there is a light ahead.
Support. Be supportive, positive, and encouraging. Ask if your sister or her extended family needs any assistance.