Based on a novel by Thomas Tryon, The Other was an early entry in the creepy little kids sub-genre that dominated horror movies and novels in the 1970s. On a more personal note, I remember it scaring the hell out of me when I saw it on television as a kid, and its recent release on DVD represented my first chance to see the film in several decades.
The setting is rural Connecticut in 1935. Niles and Holland Perry (played by Chris and Martin Udvarnoky in their only film appearance) are living on the family farm with their mother and extended family. Diana Muldaur, veteran of two generations of Star Treks, plays the twins' mother. She remains traumatized from an event sometime in the past, leaving her emotionally distant and unable to fulfill her duties as mother, though for a bed-ridden depressive, her hair and makeup are oddly perfect.
With their mother emotionally absent, Niles has bonded with his Aunt Ada. The two are fond of playing The Game, which Ada believes to be an exercise in imagination, but Niles regards as something more. While playing The Game he seems to find himself in the body of a bird flying overhead, and he uses it to learn a trick being performed by a carnival magician.
Niles compulsively carries a small tin box containing a ring that should have been buried with his late father and a mysterious little item wrapped in cloth. Director Robert Mulligan obviously wanted the viewer to remember the box because it is constantly heard rattling around whenever Niles is onscreen.
Niles is apparently the only one of the pair born with a conscience, with Holland usually getting both of them in trouble. When bad things start to happen like the twins' tattletale cousin being "accidentally" impaled on a pitchfork, Hollands true nature becomes evident.