It's hard to say. She may have actually had a crush on your brother for sometime and yes at her age it's not uncommon to like another boy too. Maybe she just wanted a boyfriend and the other wasn't moving quick enough. Maybe she is just wanting to get the other boy to notice that she now has someone and it's not him. Hard to say as she is a teen. Quite complicated minds. I never understand my teens, one minute they say one thing and the next they have a change of heart. Just let time take care of things and try to stay out of it and ignore him the best you can.