I Was Taking Care of My Sister's Friend, a Cute High School Girl, and Then We Liked Each Other.

The room of Tsukasa, a college student with no romantic experience, has become the hangout spot of his sister and her delinquent friends. One day he finds a bulge in his bed and after he pulls off the blanket, the most beautiful of his sister's friends, Erika, is sleeping there. "Oh sorry... I'm very sleepy because of my period." "You're too naked to respond correctly..." Not being able to leave a naive and uninhibited girl alone, Tsukasa teaches her some manners and common sense. Erika, who fell in love there, doesn't hide her feelings of "love". Whereas Tsukasa's chest tightens up, as he sees her studying hard... So begins the love life of a delinquent high school girl and a college student, who love each other!

(Source: Fantasia Bunko, translated)