Other Than My Friend's Sister, Something Else summary:

Seula has endured a long-standing crush, thinking that being friends would be enough. But when she learns Areum has accepted her boyfriend’s proposal, she breaks down. To soothe her broken heart, she gets drunk and calls her to confess her feelings. The next day, she wakes up naked next to Areum’s sister, Daun! When Seula begins to piece the pieces of last night together, she begs Daum to keep this a secret from Areum, to which Daum agrees under one condition… “Want to keep having a good time with me?”