Maybe try to talk with you best friend! She is you best friend for a reason. If she seriously wants to end your friendship over her brother, you need a new best friend. Talk to her. Talk to her brother after. Maybe if you tell your friend, she could also help you let him know that you like him! But if/when you talk to her, make sure to say that you don't want this event to get in the way of your friendship, and you don't want things to be weird. (you don't have to do it this way, but that is what I would do.)
