Hatsukoi Nante Shirane yo!!

From Chara Selection:

Elo, age-unknown, secretive-looking man -- Keigo, a college student who fell in love with an older niece who had never felt a passion for a woman, skipped lectures and missed her every day! But she's actually an American university professor, and when she confesses to her first love, she returns home!? And she finds out that she's her father...!?

Official: Chara Selection, Tokuma Shoten, Piccoma