Look brides I hate to break it to you but your wedding is NOT the biggest deal ever. If you think you are bestowing the biggest honor on someone to have them join your wedding party (especially late) as a way of making amends for something you did.... stop just stop!
Take them out to lunch. Buy them a cute coffee mug. Maybe even appologize. Anything but saddling them with a responsibility they don't want.
My cousin wasn't doing any favor for me. Suddenly I had undue pressure placed on me. Because if I said no I would be the ***** not her for going back on her word. I had just recieved expensive shoes I had ordered to wear when I thought I was just a guest that were now not going to match what she was telling me to wear as a hostess. I had to ask for extra time off of work to make rehearsal. And my boss was suspicious of the tming considering I had told her about needing time to go to the wedding as a guest months before hand.
It just wasn't fair to be left holding the bag for someone elses mistakes or problems or whatever.