I am a cancer survivor. While I was not so private about it at the time I was undergoing treatment for lymphoma, I was very tired of being considered first for my disease. Sometimes I felt I could see it in people's eyes, or not. Some would not even look me in the eyes. I would have loved a quilt, but not one that spoke first of my cancer. It's a hard call to know how individuals react.
I have two friends who did survive breast cancer. They are private too and prefer not to dwell on the past. A quilt with such a public icon would not suit them at all.
You know your sister better than any of us and your thoughts or doubts are more relevant than our opinions.
The most important thing, it seems to me, is that you are making this quilt "backed" with love.