I don’t remember if the siblings were both girls, boys, or one of each, but in the story, each family always had two children, and the younger sibling was always sacrificed in a ritual or ceremony.

In the webtoon, the main character somehow escapes or survives the ritual that was meant to kill them.

Might be The Monstrous Duke's Adopted Daughter. Details don't match what I recall but it at least has the sacrifice.