Satoshi is the older brother of Satoko Houjou. Although he is currently missing, his past influence is the key to the actions of several characters in Higurashi. Similar to Satoko, he cannot distinguish the difference between cauliflower and broccoli. Because of this, it can be inferred that both Satoshi and Satoko are color-blind. Satoshi is referred to by Satoko as Nii-Nii, a childish form of the Japanese word ani (兄, ani meaning older brother).
SPOILER : It is revealed in Matsuribayashi-hen (Festival Accompanying Chapter), that Satoshi is in fact alive. Shion Sonozaki learned from Kyosuke Irie, that he has been kept in the underground levels of the Irie Clinic. Satoshi is sedated and restrained, undergoing treatment for Level Five Hinamizawa Syndrome, which surfaced after he had killed his aunt on the night of the Watanagashi.
(from Wikipedia)