The heroine wants to start a relationship with her beloved stepbrother.
When the heroine was young, she got a new stepbrother when their parents got married.
She always thought that boys were mean, and at first she didn't want a stepbrother, but he was kind and protective of her and she gradually accepted him.

Recently, she finally realized that she likes her step-brother in that way... but even though they're step-siblings they're still brother and sister, and it's hard to hope for a relationship that goes beyond what they have now. While she was worrying about this, a friend told her something that changed everything:

"Stepbrothers and sisters can get married y'know?"

Hearing these words, she made up her mind to create and execute a plan to seduce him. Fait accompli!