There’s No Way My Brother is a Dark Magical Girl

My brother’s phone is full of magical girl manga—guess he’s really hit puberty, huh? As his little sister, I can understand that he might keep a rare, priceless magical girl costume hidden in his closet. A special quirk of his, right? Sure, as his sister, I can accept that.
But when my brother actually transformed into an evil magical girl and showed up on the battlefield… that, I couldn’t understand at all! Just when did my silly brother fall to the dark side?!