Yarisugi! Katei Kyoushi no Onee-san

When a college student hears that her cousin, our protagonist is worrying over his college entrance exams, she take charge of giving him home schooling, teaching him humanities and sciences. Our protagonist is filled with lots of anxieties, however he asks her for help. However what she's about to teach him doesn't just include study... [Edited from DLsite English]