Wedding Faux Pas . . . Maybe?

So . . . I finally figured out how to fit into the god-awful bridesmaid dress I have to wear for my brother's wedding. (Trust me, I'm not just saying it's awful. It REALLY is! Even the bride, once she saw it on her Matron of Honor wasn't so thrilled with it!) I just had to rip the under skirt out.
The good stuff about the dress:

• it fits
• I don't have to wear a bra

The bad:

• BOOBIES! (Not that it's necessarily a bad thing, but when I'm showing more boob then I did at my wedding (and that was a lot), it's NOT a good thing)
• It's UG-G-G-G-LLY!
• The dress makes me look 9 months pregnant (Okay, maybe 7 months, but STILL!)
• Three of the 4 of us are wearing purple, we're going to look like the friggin' California Raisins!

source

Okay, maybe not as wrinkly. But still! YEEP!
And my darling Italian won't let me go get the skirt altered so it looks better on me. (Grrrr!!! BAD ITALIAN!) Though I understand his point of not spending money on something that I'll wear once for an hour.

An hour? That's it?
Okay, so here's my plan . . . the day of the wedding, I'm going into Nagshead to the beach house and get changed into the California Raisin dress. Meanwhile my parents are taking my Italian out to breakfast and I'm going to give Mom my reception dress.
I'm planning . . . after the ceremony, pictures, and all that formal schmormal stuff that you have to do with weddings is over and the reception starts, I'm slipping out of that dress into something that:

• looks good on me
• is comfortable

Is that a faux pas? Is that a bad thing? I mean, I know, agreeing to be in my brother's wedding, I have agreed to wear an ugly dress to honor my brother's happiness. But . . . really? The reception too?

Dear etiquette-wise peeps . . . besides running the risk of an upset brand-new sister-in-law . . . am I breaking any "rules?"