Probably spoiler ahead----------------------------------------------------------yes there is incestuous relationship involved. Blood-related between two brothers. Its not the main pairing, but if you're interested to see where its started, it is on chapter 7. Where the black hair MC went to meet his long lost older brother. Honestly saying, both of them are mutual feeling, but of course, because they're brother, it is not possible.
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Sayonara Boku No Princess Lolita