Nanami to Konomi no Oshiete ABC

You're a college student on summer vacation. In your neighbourhood live two extremely friendly cute twin girls, Nanami and Konomi Nishimura. One day their mother asks you to tutor the two of them. "Nanami wants to study with onii-chan!" "Onii-chan, will you teach Konomi?" Of course, onii-chan immediately replies. "Sure, if you want me to, it'd be my pleasure." "He didn't say no... thank goodness." "Yay!" And thus begins your summer "lessons" with Nanami and Konomi. [Translated from the Getchu description]