Sae Oppawa Ireomyeon

When being introduced to her mother's boyfriend, Lee-seo is embarrassed to learn that the man who will become her new brother is Geon-ju, with whom she had a one-night stand the previous day. It was a one-night stand that he couldn't remember because he drank too much, but Geon-ju was only looking at him with interest...

(Source: Lezhin, translated)

Note: Part of Bomtoon and Lezhin's Baedeong Danpyeonseon (배덕 단편선) Collection. Chapter count includes a prologue.