Gumiho, please save me!

There is a rumor that if you live on an island where a gumiho lives for a year, you will find an incredible treasure. Many people heard the rumor and went to the island where the gumiho lives. However, no one returned alive, and the female protagonist who went to such an island. She came back trembling with fear, but what she saw was not a gumiho, but a pretty older sister??