Shinseki No Ko To Wo Tomadirakara -

親戚 (shinseki) in Japanese means "relatives" or "extended family." It refers to family members beyond the immediate nuclear family, MailMate.jp What is shinseki? - MailMate

(Shinseki no ko to, tomadotta kara…) “Because I was confused with my relative’s child…” shinseki no ko to wo tomadirakara

The narrative typically follows a familiar "forbidden encounter" setup common in the genre. It centers on a male protagonist who ends up staying overnight with a female relative—often a younger cousin or niece—due to family circumstances or a holiday visit. 親戚 (shinseki) in Japanese means "relatives" or "extended

Note: Some viewers warn against reading the manga adaptation, as it is considered a poor representation of the serious tone found in the anime. shinseki no ko to wo tomadirakara

It might be:

「おじいちゃん、見て！ この子、すごく可愛いでしょ？」 声の主は、隣に住む叔母（おば）の息子、拓也（たくや）だった。 彼の腕の中には、まだミルクを飲み終えていない、丸くて柔らかな頬の子どもがいた。