It's a bit convoluted, but Yuel and Tavi were originally created for one of my other VNs, All Ashes and Illusions. Yuel is the main character in Ashes and Tavi is just a side character who doesn't have much screentime; the two aren't in a romantic relationship in Ashes and it wasn't something I planned on when I was putting Ashes together.
While I was writing Ashes (and making myself sad because it's a really depressing story) I realised Yuel and Tavi's interactions were a lot of fun to write and they had a lot of chemistry.
Since Ashes was such a miserable story to work on, I wanted to stick my boys in another universe where they could be happy, and Yuel and Tavi wound up being a couple because they have such good synergy... But I kept them as cousins since I'd written them that way in Ashes, and it feels too important to their relationship to change.
If I'd planned for Yuel and Tavi to be a couple I wouldn't have made them cousins. Things just kind of happened this way after I got unexpectedly attached to them...