@@jeffandrobyn I'm a repeat BM (think 27 dresses, no joke). I've been in BPs that are quite large, and BPs that are quite small. This is YOUR wedding, your day. It's a sticky situation because she's technically family (or will be). Nothing makes a wedding process more terrible than an awful BM. I'm sorry. I wouldn't walk on eggshells for her because that will make it more stressed out, but I would just keep her on a need to know basis. Ie: THIS is the type of dress, color you will be wearing, if you don't like it, feel free to step down.
Good luck!