Undryable Younger Cousin

So-jeong, who lives with her older brothers, is living as a sex partner with her younger brother, Sang-woo, without her eldest brother Min-woo. However, I'm worried that Sang-woo's reaction isn't cool these days. Sang-woo started seeing his ex-girlfriend again. Meanwhile, Min-woo witnesses So-jeong and Sang-woo having sex one day...