It sounds like they probably saw something on the mammogram that they want to check further, especially if this is your first mammogram. This will be your baseline mammogram, one that they compare all future mammograms to in order to look for changes, so they want to be sure they know what everything is. The ultrasound is what they use to determine if a finding is solid or fluid-filled, so it is pretty common in a follow-up. I don't know what the "call-back" rate is. When I had my first mammogram, I had negative results for two cancerous tumors. I had one "call-back" on my remaining breast that was described in the report as a "mass," but turned out to be nothing in the follow-up mammogram with compression views. Something like 80% of all biopsies performed are "benign" findings; on the other hand, I wouldn't ignore the situation either. Call-backs are scary for everyone! And with your sil just diagnosed, I'm sure you are especially fearful. A good friend here would say, "try not to worry 'til you know you have something to worry about!" Easier said than done, but good advice, nonetheless! Let us know how it goes.
"There's a difference between a philosophy and a bumper sticker." --Charles Schulz