Q: I asked my soon-to-be sister-in-law to be one of my bridesmaids. She wasn't thrilled, but agreed to be a maid because it was important to us. Ever since, though, she has been difficult. She called my fiance crying last week about how much she hates the bridesmaid dresses I picked. How can I deal with this?
A: A demanding bridesmaid is never fun, but when she's part of your family -- ouch. You need to start by deciding what your priority is. You say you've chosen the dresses, but are they ordered? If you're not that obsessed with having your maids match perfectly, you could consider allowing her and all the other bridesmaids to pick a different style.
If you really want this dress, you need to make your case -- tell her the reason you like the dress on her. You might also recruit your fiance just to gently reinforce your POV.
Beyond the dress issue, the key here is to pick your battles. If she's fighting you on everything, you can't expend all your time and energy (especially when you're already bogged down in a million wedding details) dealing with her. On top of that, she's family, so you can't completely blow her off.
Yes, she's definitely being a pain, but don't let it bring you down. Deal with issues as they come, and remember, you don't have to come out of this as best friends. Focus on what you and your fiance want, and remember that you're building this family together (including his sister).