So in my fantasy novel-thing, I plan on having the main character's older brother join the villain's cause. He left to join the army early in the first book (which i'm still working on), and the next time we see him he's already a key member of the villain's force. Originally I was going to explain through some flashbacks during the fight, but as I thought about it, I thought that it would be more effective for his decent into evil to be a story along with the MC's story. But I'm unsure because I feel like that it might take away the reader's attention from MC's journey throughout the story. Also if I do make it a sub-plot/side-story, it loses all shock-value for the reader I think. (Although who ISN'T expecting a member of the MC's family to be a bad guy, is the real question) Anyway I'm still not really sure. Any advice/suggestions?