Dress advice.. Help please

Onnilee • Adalynne Grace 09/11/2016 🎀💕👑 Tinsley Saylah Joy 04/24/2019 💜🌸👑 Evangeline Elizabeth due 04/02/2021
My sister is getting married a month after my baby girl will be born. She chose the purple dress for her bridesmaids to wear (First picture). So seeing as I will be a month postpartum what size should I order? I wore a dress size 8 before I was pregnant. Second picture is me 33 weeks pregnant. The sizes for this dress are s,m,l,xl,xxl, and xxxl. What size should I order for my sisters wedding??