Women are the best 2 faced creatures on the earth. She can be your best friend to your face and tear you apart behind your back.
I would bet she is the one behind it and your brother is like MOST males, ***** whipped.
Sorry buddy but if he marries her you are out more or less. You need to have a sit down with your brother to get to the bottom of it but also even if she was not doing anything since he has found someone he will be spending more time with her and that's less time with you. It's just life.