How to Say No to Being a Bridesmaid

My boyfriend of seven years has a younger brother who recently proposed to his girlfriend. She's always casually mentioned me being a bridesmaid when referring to her future dream wedding. I dread having my photos taken and know I would feel so self-conscious on their big day, not to mention all the bridesmaids' duties - dress, hair, makeup, trials and days slash nights out. Am I okay to say no to being her bridesmaid or am I in the wrong as I know their wedding day is nothing at all to do with me? It is their big day and I know even asking this might sound selfish.