The heroine wants to seal the deal with her beloved stepbrother.

Ringo has had a stepbrother since childhood due to her parent's remarriage. At first, when she still had a strong impression that boys only did mean and nasty things, she wouldn't acknowledge him as her brother. Nevertheless, her kind stepbrother Kei gently protected her and that swayed her heart.
Recently, she's started to see him as someone of the opposite sex... even as she's aware of their status as stepsiblings, and that hoping for a relationship that goes beyond what they have now will be difficult.

"We're only stepbrother and stepsister, I can still marry him."
With these words in mind, she gathers her resolve for the sake of making it a done deal, and carries out her plan to seduce him.

(Source: DLsite garumani)

Official Translations:
English, S.Chinese, Korean

Note: Originally published as one book, later split into 2 chapters and published by Night Comic.