Need advice please!

My step-sister is getting married 5 weeks after I am due. (May 28th) I am one of her bridesmaids. I am trying DESPERATELY to forget about my feelings and focus on her and her big day. But she is planning the wedding in 4 months. Was adamant about having it in May and I tried asking if we, as bridesmaids, could wear a dress that wouldn't make myself feel so awful about my after-baby body. She chose a tight and short dress for her bridesmaids despite my asking her not to and even grabbed the size she thought I was. So I'm stressing about feeling so self-conscious in it and that I may not even fit it in it! Am I being a brat? What should I do?