D.B.
I used to have something that sounds like this, except it was multiple lumps. Same thing - dense breast tissue. They called it fibrocystic breast disease - benign but painful and affected by hormones. In my case, it often looked suspicious on mammograms so I had to have a lot of repeat scans as well as some biopsies. They were all negative but it was stressful and expensive. They didn't go away when I was pregnant although they kind of got mixed in with the pregnancy-related breast tenderness and swelling. Since I started supplementing with a breast-health product, the cysts have disappeared and even my doctor can't find them. She's thrilled with what I'm doing and said to keep going because it's safe and also a good preventive for osteoporosis. Happy to help you and your sister if you want more info.