Akuma de Sourou
Kayano is a normal high school girl. She has a crush on a charming guy and has caring friends. She seems happy. All of that is about to change when Kayano's world is turned upside down. After loosing a love letter (meant for a schoolmate) the scary Takeru finds it. He then blackmails Kayano into being his slave using the letter. That's just the beginning! Kayano's mom is remarrying, and guess who, Takeru's dad! Kayano and Takeru will now be brother and sister. Can things get any worse? ...Oh, believe me, they can.
Some aspects of this series actually reminded me of Hot Gimmick (e.g. the whole slave thing), but really this series is quite different. I think it is a very enjoyable manga. I think it is also pretty realistic. Step-siblings falling in love is a constant subject matter in manga (ie/ Marmalade Boy, Random Walk ect.). I find that this series stands a part because the characters really contemplate what they're doing, worrying it may seem wrong since they're "family". I liked this series.
Akuma de Sourou was created by Mitsuba Takanashi. It is 11 volumes long. I don't think there has been an anime series made. It has not been published in English.
There are some more mature themes. I would rate it around 13+.
Kayano- The main character. She is strong and caring. She also gets confused a lot... -_- Kayano is continually crying over one thing or another, usually blaming herself for others worries. She always wants other to be happy, but will do anything to be with Takeru.
Takeru- He acts coldly, but is actually pretty kind. He likes to tease Kayano, but you can tell he cares about her. Takeru was a delinquent in the past, but has calmed down. He holds a grudge against his mother, who he feels doesn't want him. He blames himself for his parents divorce.