What does that have to do with you're light skinned brother? He would also be black, that woman is black as well. She is black because she is of African descent, she is just as black as you, me or members of my family. If I see someone who looks like me why would I not call them black? I'm black, I'm not 100% African-American by any means.
She is whatever she identifies with, that is her choice you do not get to decide that. Your logic epically is flawed when it comes to people of mixed race, like myself. You do not get to decide who identifies with what, just shut up and deal with how they feel about themselves.