A favor
This person found out that their own cousin was in fact their sister, that is because of a favor that her aunt and uncle did for her parents long ago. Her own parents could not have kids, so they asked them if they could help out, they accepted, and months later, she was born.
But instead of the uncle and aunt raising her as their own, they made a deal and gave her away to the people who she knew as her parents. Finding out that your aunt and uncle are actually our mom and dad must be a weird feeling, but in some way, it is a nice story.