User blog comment:Zathronas/Analysis by an author: Volume 2 episode 6: Burning the Candle/@comment-25091485-20140905022200/@comment-24218038-20140905024856

@ Nitefallen

Yang clearly state she learned of her mother after Summer's death.

Qrow could be the brother of any of the other member. But your speculation makes sense and I believe he is Summer's brother.