Yamijima Island is a main location from the Forbidden Siren 2, the place where entities known as the Mother and Otoshigo reside. Siren: New Translation (Blood Curse) confirmed existence of the island within remake canon. Some island’s famous locations can be found in a posters in Episode 1, in the house on the hill. It can also be a Easter Egg and indication that house owners were visiting Yamijima at least before 2007.
Island‘s fate in 2007 (Blood Curse Timeline) and it’s background is currently unknown.
Note: Remake canon and timeline not to be confused with original Siren games canon and timeline. Same goes for Yamijima Island within New Translation canon.