This is a hard situation. I'm guessing she is upset not because she is not in the wedding, standing up for you, helping you out, planning our bachlorette party, bridal shower, etc... She's not there for you when you are having a rough day, or when you and you FI are arguing over what color the napkins should be. It sounds like to me that she is living in a fantasy world that since she is your sister she gets to play BM for a day and look pretty. But not except all the other roles. I think that asking her to read a poem or something is a great idea. And after that you have to move on