I have really been thinking about this subject, and I have changed my mind back and forth several times. I belong for a support group for macular degeneration, and, even though I know it is a different thing, most of the members stress that "they are more than just the disease." If that transfers over to breast cancer, the pink ribbons fabric really may not be the right backing for your sister's quilt.
I am going to second bigsister 63, if you are in doubt, use another fabric. (Just my thoughts on it all....) You are so thoughtful, not only to make the quilt, but to try to really really please your sister.