The thing is, in Japan there's no obstacles legally for adult (blood-related or not)family members to be in a sexual relationship(whichever form it takes, mother/daughter, father/son, brother/sister etc) provided they're of age, legally sane to give consent and not under any form of guardianship by the other at that time, the only thing they can't do is get recognized in marriage or in any other official partnership or have children(as far as I've understood Japanese law, correct me if I'm wrong). Which might explain a little why this is 8 volumes and 9th pending(I like this, I like this a lot, a yuri and incest fan myself, but, but... 'looks at all those other interesting mangas cancelled/axed')...