This is a highly colloquial, somewhat modern regional or internet-slang ending. It adds a tone of "you know how it is" or "and so, well..." to the sentence. The "Relative's Child" Trope in Media
To understand why this phrase resonates, we have to look at the individual components of the sentence: shinseki no ko to o tomari dakara de na tesa
The protagonist’s parents often leave them alone with a younger cousin, forcing them to navigate the challenges of being a temporary guardian. This is a highly colloquial, somewhat modern regional
The phrase (親戚の子とお泊まりだからでなてさ) roughly translates to "Because I'm staying overnight with a relative's child..." in Japanese. While it sounds like a mundane excuse or an explanation for a sleepover, it has gained traction in specific online subcultures, often associated with anime-style storytelling, roleplay, or light novel-esque scenarios. The Linguistic Breakdown often associated with anime-style storytelling