Chakgang Mate

Drunk, Eun-Seong nearly had sex with his long-time crush, Jeong-Hyeop, but mistakenly believed the person was Jeong-Hyeop's older brother, Jeong-Woo. Jeong-Hyeop, seeing Eun-Seong acting as if nothing happened, falls into another misunderstanding, thinking she likes Jeong-Woo instead...

Childhood friends' mutual crushes making a complete mess of things.

(Source: MrBlue, translated)

Note: Includes one extra chapters.