They're not related by blood at all. Everyone get on board this ship safely now.
Personally, I don't mind reading an incest story especially a historical one since throughout history, a lot of royal siblings did have sex and all that stuff. Anyway, I only mind incest in real life, if it's fiction then there're practically no taboos that'll bother me. But I'm happy this was revealed because if they want to get together, later on, it's much more appropriate if they aren't related by blood. Although as the MC said, the emperor is the law so I'm sure that if the ML becomes a strong emperor, everyone would shit themselves and no one would dare question him even if he marries his real sister