Thanks for all of your encouraging comments.
I finally talked to April (my best friend, a bridesmaid, and a sister), and it made me feel a lot better. We came to the consensus that this was a good thing. This ex-bridesmaid has tried to steal the show with one stunt or another the whole time. This is just another one. Blah...who needs that kind of stuff during a rehearsal dinner and wedding?
I don't think it would be a big deal to have two groomsmen walk up the aisle with a bridesmaid. Or one of them could walk my mom up. When I told April that I was toying with the idea of asking someone else but kind of wary to do so...she said (and I quote), "Dude, that is what you joined a sorority for. Ask any of our sisters that you think would fit in the dress. None of them would be offended, and more than likely, they would be so pleased to help."
I have an idea of who I'm going to ask, and I think I'm going to try and get in touch with her this evening.
Cross your fingers for me!