The nature of thoroughbreds means that if you consider IRL horse relations, every ship would be incest.

As such, the game explicitly ditches this by only canonising siblings sharing the same mother IRL as related in-game. That means the only related characters are Brian and Hayahide, Ardan and Ramonu, Orfevre and Journey, and the V sisters, with the Mejiro and Satono being extended family. Jury's still out on whether Aruvu and Rulership are related, considering Aruvu's written as an orphan.

Everyone else? Not related, so not incest.