What would you do about your fiancé’s sister in your wedding?

What would you do about your fiancé’s sister in your wedding?

"I’m getting married this Summer and I'm having issues with my fiancé’s sister. She a bridesmaid, and she hates the color and design of the dress I picked out and she doesn’t want to wear it.
She’s been so difficult, but since she's my fiancé's sister I don't know what to do about it."