Oshi ga Imouto ni Narimashite.

Ichika Kojima is a high school girl obsessed with the idol "Kanon-chan." Ichika is beautiful and popular among the younger students (girls), but she gives herself over completely to idol-otaku activities. When her dad remarries, she ends up with a new stepsister. It turns out to be "the little sister of all humanity," her beloved "Kanon-chan" (real name: Akane). A hyper-yuri love comedy begins!

(Source: Gentosha Comics, translated)