Oshiegoto: Doujin Kyoushi to Tensai Gal

Sachi Kurokawa, a doujin artist in her private life, is hired to be the new art teacher at a high school, where she meets Koihaku, a gal who happens to be a talented portrait artist. Koihaku is a fan of Kurokawa's doujinshi work, and her praises make Kurokawa happy, but trying to keep her doujin work a secret, she lies and tells Koihaku that it is her older sister "Sachi" who draws the doujinshi. Now Koihaku wants Sachi to teach her how to draw like "Sachi"?

(Source: Akita Shoten, translated)