I love the smell of incestual love in the morning
"irreplaceable blood-related little sister"
Why did he phrase it like that?
It does sound abit awkward in English. But I can see it possibly being less awkward in the original. Some choice of words can feel weird or unnatural in one language while being nothing weird in another.
Another aspect that may contribute is because how lengthy the sentence in the English translation. 'little sister' is just one word in Japanese, and IDK what 'blood-related' in Japanese is but it possibly shorter.
Anyway, his wording here because in just before Reina said how they are just being blood related and barely sibling because they don't grow together, a statement that can be taken to imply that blood relation isn't worth much in itself*. So, he mention blood in his response to stress that said blood relation is indeed worth a lot for him
Which make sense when you consider Yamato's background. Even before all the monster thing, dude is protective of his family, working hard to protect his brother and even then still wishing his then-abusive father to come to sense. And then just right after he lost both his little brother he spend so much effort protecting as well as his father who just had to started getting better. Make sense he is driven to protect the only family member he has left.
*not that Reina seems to think so. Its very obvious she's happy to met her brother again. Those sound more like words of self-denial.
Anyway, if they have grown apart since very early on, it kind of explain all this blushing reaction. No time for Westermarck effect to develop
