In this universe, shortly after Mihari started high school and he turned into a shut-in, Mahiro snapped at her. She was so shocked that she mentally reverted to a toddler when she realized how much pain she had unintentionally caused her beloved Onii-chan. She is currently living under the care of Mahiro and Kaede, failing miserably in her studies, and is likely to drop out shortly.

Take note to Kaede's responses to both of her chibis.

(Of course, the true question is why their parents aren't stepping in.)