Folklorist's Note 10 is an old book in Fatal Frame II: Crimson Butterfly.
|Twin Shrine Maidens
Twin Shribe Maidens are sacrificed in the Crimson Sacrifice Ritual.
Boys are sometimes used as well.
The people of this region believe that twins were once a single being, which was split into two at birth.
The ceremony is based on the belief that when the two bodies are reunited as one, the Shrine Maiden will gain the power of a deity.
The text says "the older sister must XX the younger, and throw her into the X".
The XX part must refer to the most horrible part of the ritual, probably some kind of "sacrifice".