Remnant: Juujin Omegaverse

Daath lives at an orphanage with his younger sister Bella. Unlike other children, as omegas, once they get their first heat, they will be married off. Daath is determined to make sure that he will not be separated from his only family, more so once he finds out that the priest has been selling the omegas off instead of having them marry. When Bella suddenly goes into heat, things come to a head, and only the mysterious alpha beast-man Juda tries to help.

(Source: MU)