M.P.
I suggest that you make an appointment and go with her to talk with the doctor. You may feel more comfortable with him and his diagnosis and treatment than your sister does. She's young and may have not handled the information well. I would be quite scared if I were her and also may not have listened well.
It's reasonable to get a second opinion. I'm just suggesting that you know more before going for it. You can always ask for a referral to another doctor who is a specialist in this field. It seems like that this doctor would welcome that. The medical profession actually suggests getting second opinions.