When I was a bridesmaid for my friend I was away travelling during the planning stages of the wedding and couldn't believe the way the other two bridesmaids were behaving.
They were refusing point blank to wear dresses from certain stores, one even said she would only wear a dress from coast which cost £250 - although she made no offer to pay for the dress herself! The same girl also refused to go to the wedding unless her three young children could go - even though my friend had said she didn't want any children at her wedding (let's be fair, who wants to pay £100 a head for a child who's not going to remember it, and is more than likely to cry during the ceromony (which they did)?!).
I think these things are always hard, but I would suggest if you're going to keep her as a Bridesmaid, then don't involve her in any of the details of the planning. She doesn't need to know any of the finer details, so that should hopefully give her less of a reason to stir if she's kept in the dark.