Yui's manga / #Incest(16)

Ah it could have been a favorite but i'm confused. I don't mind incest that prohibition came due to genetic abnormalities in the next generation and it's not like they can get pregnant but reading chapter 6 I had the idea that Haru's the real son and Tatsuya's not. I don't know why I got that idea since Tatsuya's place was never questioned but that's how the dna test result looked like for me. It's no biggie as I've said I don't care about incest or not but it's a big plot hole for my peace of mind.