Uhh, what? Illya is a homuculus. Just because Kiritsugu is her father doesn't make her not a homuculus. Fact is, Kiritsugu didn't even make Irisviel pregnant, because, as a homuculus, she can't get pregnant the normal way. What happened is the Einzbern's took some of Kiritsugu's DNA and artificially inseminated Irisiviel with their alchemy and that's how she gave birth to Illya. Illya has some of Kiritsugu's genetic material in her but she is 100% homuculus.