.
A brother might be attracted to his sister because of the simple reason that he is curious about the opposite sex and therefore it is causing him to look towards a source that is familiar and comfortable from which he can learn about it.
When Sousuke imitates Kurz hoping to cheer her up after his apparent by jokingly propositioning her, Mao just delivers a before giving him a noogie.
Also doubles as a since Sawada.