I believe you, yourself, need to make this decision. You know your sister, and probably know deep inside how she would feel about it.
I can't imagine how I would feel if I were in her (or your) situation.
Seems like the flannel honors both the battle of the victim, as well as the awareness of cancer and the hope for the future.
Maybe I would not use the pink ribbons, but write her a heartfelt letter of how you personally love her and how you hate this enemy that has taken such a toll on your family.
My thoughts are with you on this.
Linda