Hanazono-san Chi no Futago-chan

Tatara Tani is an average high school first year, both in terms of grades and athleticism. He has two childhood friends that attend his school, and they're twins! The elder sister Ranko is a flashy gal, while the younger sister Yuriko is a popular idol. They had slowly become distant from Tatara, but even so he always secretly held a crush for Yuriko. One day, Yuriko suddenly asked him out on a date. This date was the beginning of a secret relationship between the twins and him!

(Source: Kodansha, translated)