The phrase "Shinseki no ko to o tomari dakara" (親戚の子とお泊まりだから) roughly translates to "Because I'm having a sleepover with my relative's kid".

: As an adult production, the plot serves as a vehicle for explicit content centered on the "staying over" premise. Shinseki No Ko To O Tomari Dakara Studios : dry-goods

I see you're interested in a guide for "Shinseiki no Ko to O Tomari Dakara de na! Jikū Top"!

And for the first time, the hallway between our rooms felt a little shorter.