I also noticed "smooth margin" when I read your message and know that is the preferred margin so, I agree, that is hopeful. Hope you can talk with your sister soon to interpret all of it for you.
You may have a bad feeling just because this is the third time you've been called back. I hope saying this will be reassuring, but even though I eventually was diagnosed with breast cancer at 54, I had biopsies and ultrasound callbacks going on for 27 years first. At 27, I had a biopsy and it was a benign cyst. Almost every other year I had a mammogram then ultrasound when they thought they saw something, but the ultrasound changed their mind. The first time I was called back for an ultrasound I was anxious. In the years that followed, I just expected that routine. You may have to deal with callbacks every time that end up being harmless stuff. Good to know your providers are erring on the side of caution. I had fibrocystic breasts which always muddied the waters for exams and imaging. Good to keep up with all your routine checks. The waiting game is the worst. Praying for good results for you.