Kimi wa Sekai de Hitori no Omega

Sae is a college student with a crush on her cousin, Hasumi, who works as a researcher at the same college, and has been like an older brother to her. Despite her intentions to keep her feelings a secret, her body goes into heat, which is a phenomenon that was thought to have disappeared long ago. Even as a researcher in his thirties, Hasumi cannot think rationally in this situation. This is an omegaverse story about two people who love each other being brought together.

(Source: Renta!)