Bridesmaid Dress Dilemma?

Okay, so my brother is getting married in 2 weeks! It was just umed that I would be a bridesmaid, they never formally asked me. the wedding party is his fiance's sister and his bestman. then me and my husband. I have never supported their relationship because they have issues but I decided to order the stupid dress even though it would be horrible for my shape. I have a larger bust and she chose a strapless dress that makes me look like a hooch and bigger than I am. She didn't even ask my opinions or anything. I got the dress and it doesn't even fit and they don't have time to do alterations. I just left a message with my mom to spread the news and I told them that I don't feel comfortable wearing the dress and I do not wish to be a part of the party because even if the dress did fit, it would look horrible. today is probably going to be a stressful day. am I the devil for "ruining" her wedding or is she a narcissistic person that just cares about herself because she never wanted to take any opinions or even have the courtesy to ask if I would be in their party?