Reiko was sacrificed.
As revealed in a bonus chapter explaining how Yuina and Reiko met, she found Reiko alone at a shrine. Given how she never left it until Yuina voluntarily adopted her, it can be assumed that Reiko was tethered there. But what event could have happened to make her unable to leave? The likeliest answer is that the shrine used to oversee human sacrifices with Reiko being one of the potential victims. That would also explain the apparent amnesia Reiko suffered: the trauma of her death was so horrendous, she blocked it out of her mind. Even then, that would not stop her from crying regardless of whether she understood why she was.
