Well it's technically only one sided ╮( ̄▽ ̄)╭
1.The only aspect of Tatsuya that is perfectly natural — the only emotion that remains — is his love for his sister.
2.While he cannot feel lust, he can still feel aroused by the actions of certain women (e.g. Mari, Mayumi, and Miyuki). He also acknowledges the charms of females, including his sister.
3.Tatsuya became Miyuki's Guardian, so as to give him an attachment so he wouldn't become a person who would relentlessly destroy the world.
4.When Maya announced Tatsuya and Miyuki's engagement, Tatsuya was freed from the position of Miyuki's Guardian.
and it doenst matter bcuz is a manga.
In novel??
But no in manga??

this people who writes this kind of manga take incest so lightly... the more I read the more I'm also starting take it lightly....haiiiiiiiiiiiiiiiiiiii this are BROTHER and SISTER falling in love guys remeber!!! omg! when you think about it's so messed up!!!