Eien no Miyako

Hero is a college student who attends university in Tokyo. He lost father at young age and now lives with mother, father-in-law and sister-in-law Mariko.

A strange letter arrived from Mariko's pen friend Keiko who spends summer vacation on a "September Island (Ku Tsutou)".
"Strange events have been happening, please come quickly. I'm scared".
Hero used to visit this island together with his deceased father and together with Mariko they set off to help Keiko.
Keiko was supposed to work as maid in a western mansion, but only three mansion owner daughters are found there.

[From vndbreview]