(親戚の子とお泊まりだから) translates to "Because I'm Staying Over with a Relative's Kid". It is a highly popular, mature Japanese independent animation (indie anime) project created by the circle Awakoto-ya (あわこと屋).

The title suggests a premise where a character (often a younger relative) is staying over, and the protagonist must hide a secret or navigate a delicate situation to avoid being caught. Forbidden Relationships:

In the story, this eerie, ritualistic children’s song is a harbinger of danger and a key to unraveling the village’s dark secrets about karma, Cantus (psychokinesis), and the fate of "defective" children.

Here is an informative text regarding the song and the artist.