Chiyo and Chie are twin children who lived in the mansion before the events of the game. Their genders are never stated in-game, but there are details in game that would suggest Chiyo is the female and Chie is male., due to their different reactions to the mermaids.
Chiyo's room is on the second floor of the newer mansion, first door on the right as you come up the stairs. In her room you will find a diary, a toy and a dead goldfish. The diary entry is the standout part of this room, along with the half of the planet painting. The entry states that the 'fishy-fish' are miserable and her drawing in the piano room is of the mermaid with large dark eyes and mouth, crying. It can be assumed that, being female, she doesn't share her brothers love for the mermaids. In one of Rin's dream sequences we learn that Chiyo feel sad for the fishy-fish for being locked up and sad.
It is stated in an article found while searching the mansion that Chiyo died at the age of 7 in a drowning accident together with Chie and their mother Saeko Kawamoto (29) in August, 1972.
Chie's room is on the second floor of the newer mansion, second door to the right as you come up the stairs. His room is basically a mirror of Chiyo's room, with the other half of the planet painting. Although his diary entry is essentially the same, certain words has been replaced. He says that he loves looking at the adorable fishy-fish.
In his room you will also find a passage containing documents hidden behind a bookcase. His drawing in the piano room is of a simple happy mermaid. Being a male, he follows along with the other men in his family, loving the sight of the mermaids.
It is stated in an article found while searching the mansion that Chie died at the age of 7 in a drowning accident together with Chiyo and their mother Saeko Kawamoto (age 29) in August, 1972.