Kyouran Kazoku Nikki

The bullying is yet to cease, and so, Yuuka comes home crying one day, to be comforted by Ginka. The family express their concerns, especially after it appears Teika and Hyouka are ready for some seriously violent retaliation. The next day Yuuka is confronted by a teenage girl - her biological sister Senko, who has come to take her younger sister back. The truth regarding Yuuka's previous family is revealed. Her family traditionally tortures the weakest daughter of the family to force them to become demons or break them if they don't. Before the situation gets out of hand Ginka appears, and apparently knows Senko. Night comes and the bullies are at it again, this time aided by Senko, who is madder than ever before. Senko claims she needs to be saved too. Ginka's second arrival ends with Senko waking up in the Midarezaki household. Kyouka threatens to kill Senko for being an enemy of the family, but Ouka jumps in and pulls her away. After the family destroys the Himemiya mansion in massive overkill fashion, Senko officially becomes a new addition to the family - Chika Midarezaki. (Source: Wikipedia)