She leveled up and gained big new powers to seduce her stepbro with.
Honestly, Mao-chan has been cute from the start. As a kid, she was cute. In middle school she was still cute. Her problem was never about lack of attractiveness, but her stepbro trying his best to be a proper brother (and her trying to jump him as his stepsis). I like her clothes in panels 3-4, attempted seduction aside, I respect her effort to improve herself appearance-wise even when she didn't need to.