I was not familiar with the term harmartoma so I looked it up. This type of benign tumor is also known as a "fibroadenolipoma". (My type of rare b/c is closely related to a fibroadenolipoma. I had a connective tissue cancer in a Phyllodes tumor.)
Treatment is different with everyone with size of the tumor usually determining removal. The fibroadenolipoma is fatty tissue and misplaced tissue. It is a condition that usually effects 35-45 year olds. Your sister needs to do as much research as she can and ask the doctor lots of questions. The doctor should be able to discuss treatment options with her.
You are being a good sister supporting her as you are.
Two are better than one, because they have a good return for their work: if one falls down, his friend can help him up. Ecclesiastes 4:9-10