My mother has picked out a dress for my wedding next September...from Victoria's Secret!

She will be almost 52 at the time and has a track record of dressing like she's 22 (no bra, short shorts, etc.). I have told her nicely that I think it's too low cut and too short (underneath, there is a solid lining that barely hits mid-thigh) and even suggested similar, more appropriate MOB dresses, but she is determined to order it. She says that if she orders a size bigger, it will be longer and she can have it taken in. I think it's completely inappropriate. She has also mentioned that if she has a hard time walking on the grass in her heels, she is planning on going barefoot! I want her to look sophisticated and age appropriate, especially since this is will be the first encounter my fiance's family will have with her (and that's a whole 'nother story) Help!
Thanks for giving me some perspective, girls.
I don't want to tell her what to wear per se, but rather prevent anyone from talking smack on my momma! But when all is said and done, the only things that will matter at the end of the day are that I'm marrying my best friend and everyone had a great time.