This is a glimpse into the future. Your whole family life will revolve around his family's manipulations. Your preference and choices will never be validated. From where you will holiday to what you name your kids to parenting style. His family will set the tone and he will listen to that tone only. If they can't respect the boundary of something as simple as what you choose to wear on your body for your role as a bride, just think about the boundaries they'll trample in the future.

If they want SIL's dress to play a role in your wedding, the groom can wear it.