Given the content of not uploaded or deleted posts from the artist we can probably safely assume kyou (and their father) is not the father, but just about any one else is.
It's amazing that people seem to think that incest always, even with 1st generation inbreeding, leads to incredibly malformed children.
Inbreeding can lead to severe problems on several generations, but even with 1st or 2nd gens, health problems can appear, a reason why this practice is prohibited in many countries. Marriage between brothers is forbidden by Japanese law, and such level of taboo is the reason why it is used so frequently in local literature.