I don't think you would need a third opinion, unless there is something that you distrust about the information that you have already received. If your sister has already been diagnosed, she would only need to see a surgical oncologist if a breast surgery is necessary. I pray that her treatment goes well and that her recovery is smooth. May the blessings and divine healing of God accompany her medical treatment.
It is most common to be evaluated by a SURGICAL oncologist first.
The surgical oncologist will usually plan surgery after the initial appointment if surgery is the first step in your sister cancer care plan. You should select one that works closely with the oncologist that you have already seen since they willl both be involve in your sister’s long term care.
I hope that everything works out well with your sister and that my answer was helpful