So you're not actual siblings, right? Then we can totally do it.

Suddenly, Takemitsu's parents remarries, and he finds himself living with a new family, including his new stepsister, high school girl Kirika. While she acts sweet in front of their parents, she is openly hostile towards Takemitsu when they are alone, often complaining and causing troubles. She frequently brings her loud gal friends over, disrupting Takemitsu, who works at home as an occult web writer. One day, one of Kirika's friends takes an interest in Takemitsu, teasing him and even suggesting to Kirika, You're not real siblings right? So it's ok to do it then.