You didn't say how old your sister is. As you get older your internal breast tissue changes and becomes less dense that is the reasoning of not having a mammogram before 40 unless your family has a history of breast cancer. Since she had her exam and they found an area, they want to keep an eye on it to determine if it changes in size and mass. If your sister feels anything out of the ordinary - hard spot, tender spot, odd sensation from the breast or nipple area like a "let-down" feeling, run to the nearest doctor or breast specialist and demand to have another mammogram performed even if it is out of pocket to put her mind at ease.
I had a spot on a breast that was tender for about two weeks and then it went hard with no feeling and scheduled an appointment and had a biopsy performed. The results were cancer Stage I. I was scheduled for surgery (lumpectomy) and had a round of radiation with no chemo and have not had any return and that was 12 years ago.
But as another poster said, you are your best advocate for early protection and do listen to the gut feeling we mammas all have. May she just have a benign spot and nothing more in her breast. I will keep you both in my thoughts. The other S.