Why Don't You Lift Up That Skirt And Show Me Yours, Sister?

I hear my younger brother has begun searching for a certain woman: his fated one, with whom he shared a night of passion at the masquerade ball. The tiny moles on her inner thigh are his only clue... But, I'm sorry, Alto. It was I, your elder sister! This is a sweet (?) love story brought to you by a contrarian heroine who can't be honest about her feelings. Don't let the title fool you! This love is pure, through and through!

(Source: MangaPlaza)