Umineko no naku koro ni

From Animepedia

Umineko no naku koro ni [うみねこのなく頃に; "When Seagulls Cry"] is a doujin murder mystery visual novel and manga produced 07th Expansion. The story begins with eighteen people from the Ushiromiya family who are on an isolated island for two days, most of these people are murdererd, the objective of the game is to decide whether the murderer is human or not.