I wouldn't buy that dress now if I were you. You are about to go through major body changes. Who knows whether you would be able to fit into that dress by June. Tell her no way. If she wants you to be in the wedding, you'll get your dress after your pregnancy and she'll like it!
I hate being in weddings, but it's not about you, so looking terrible in a dress is part of the "fun" of it. You and your sister can make a day of looking fat in your dresses and make fun of the awful thing your SIL made you wear. Make sure you get it tailored well. There is nothing worse than a loose bridesmaid dress. You also might consider telling her that the dress won't flatter all the bridesmaids and remind her that you will have just given birth to your ___ (fill in the number) child. She may have forgotten.