My boyfriend's sister asked to borrow one of my nicest dresses for a wedding but I don't trust her with it. Is there a polite way to say no?

Sharing clothes is very personal and this is a big ask because we're talking about a beautiful and expensive dress. Borrowing a dress for a fancy event makes a lot of sense, but it comes with a good amount of responsibility. So how do you protect your clothes and not make things uncomfortable?