Another way of putting this in perspective – to the one making the request – would be to ask, “Was it your mother’s dress?”
It probably wasn’t, was it? (Although you might need to prepare yourself in case it was, and this actually is a long-standing family tradition, even if you’re only finding out about it at the last minute.)
But if it wasn’t also a grandmother’s dress, then why should you be expected to start a brand new tradition for the sake of your mother or prospective mother-in-law?