A great manga with well-think story. Its complex plot and drama makes it a "hard" work. Despite characters, they are complete assholes, the one more mature is Minami, by disgrace; but that's the point. Drama, suspense, psychological, tragedy, family issues, readable… A non-comedy Lolicon.
Its story is deep and I consider it as excellent, very strong for anyone. Why do you react if found your little sister (child) having sex with your "best friend"? This manga have a possible scenario where nobody can does anything and treat to live with that, but time cannot returns, you can not able do something anymore; and your actions in this state may make a catastrophe. Is sad and you can get some of grief and confusion over reading. One more time, is very strong. Actually, you can relay on no one, even family would betray you.
Not knowing about difference between "good" and "bad" you can't do anything.
I recommend you miss the omake, just a lost for good and strong ending. Even I didn't understand nothing of this or recognized its characters. Maybe the first extra is considered a continuation of end as well, making it more deep, and frankly realistic.
I think that characters are most like western culture, where nobody says and/or demand important things [like this] 'cause people [possible] reaction, trying to keep the family even knowing that is impossible, getting out perpetrator and don't accuse yourself (as a stupid blind), where always blame is anyone but you. I really detests this kind of people, always lying and "omit" things for sake «justice, rightness and moral», despite [and "for keep"] stability. All lies!
Lust /fleshly "weakness" and loneliness (and maybe 'friendship'), may justify each other? I think nobody can [nor should] judge no one, though…