Anata no Imouto

Yuuka has recently started going out with Kouta, the childhood friend she's always had a crush on. Two months into their relationship, they go to Kouta's house to have sex for the first time, but because both of them are inexperienced, it doesn't go well. In front of a troubled Yuuka appears Kouta's younger sister. "Become my ideal girl." A boyfriend, a girlfriend, and his younger sister. It's the start of a love triangle that's so sensual you can't look away, but so immoral that you'll wish you could.

(Source: Kodansha, translated)