Maids in different shapes and sizes?

I will have my sis, sis-in law and a close friend as my maids. My friend is "goddess sized". ( larger) My sis is tall, very thin and likes to show off her figure. I want all of the girls to be happy, comfortable and feel beautiful. black is the dress color (for fall) with red detail (like a sash or whatever). I dont care what they wear as long as the dresses all look good together, no one sticks out like a sore thumb, etc. The largest sized girl picked a really nice two-piece outfit. It has a flattering a-line skirt and a nice top with straps that doesnt show too much. She is worried because she has a large chest and doesnt want it all popping out! I love the outfit she picked. my sister says she feels frumpy in it. The only dresses she wants are skin tight, strapless with LOTS of . I dont want her to dress like this for my wedding. Its not appropriate and the other girls will feel uncomfortable next to her. Am I being unreasonable? Should she just wear it and deal with it?