that wont help, she probably will only have the money for it on the 21st and telling her it has to be sooner might make her scramble or do drastic things to get the money sooner. its better to sell it to the person who said its their dream dress (they probably want it just as much as the 21st girl) and make the 21st girl feel better by lying that you decided not to sell it cause ur family member wants it
even if this falls through I'm confident someone will buy it. it's a popular asian brand so on top of the clothing being kind of expensive so is the shipping, so when it shows up on depop it goes fast I'm going to give the first person a chance bc I'd feel terrible going back on my word. plus you can check buyers ratings on depop and she has a good one!