The phrase (親戚の子とお泊りだから) roughly translates to "Because I'm staying overnight with a relative's child." It is primarily associated with a Japanese adult-oriented anime (hentai) series.
The popularity of titles like this stems from several recurring themes that resonate with the "otaku" community: shinseki no ko to o tomari de in kara
: The narrative centers on a protagonist who stays overnight at a relative's home. The "relative's child" mentioned in the title is usually the primary focus of the protagonist's interactions during the stay. shinseki no ko to o tomari de in kara
Hungry kids are often grumpy kids. Keep them fed and happy without spending all day in the kitchen. shinseki no ko to o tomari de in kara