Haha, the answer in your head would be “yes, please” right?
If her joking was not done spitefully or in a mean way I would certainly investigate just what her feelings on the subject are. Is she laughing in a derogatory way or genuine in care. If it happens again ask her what she would think of your brother if he said yes to the bikini. You could even show your opinion by saying it would not change how you think of your brother ( which I think it wouldn’t? 🙂 )
Always try to tip your toe in the water before diving in.