The Lost Tribe (1983)

An anthropologist disappears in New Zealand while looking for a lost tribe, so his wife and little daughter as well as his shady twin brother who's in love with his sister-in-law go to look for him. Also seeking him are the police, who suspect that one of the twins is responsible for a hooker's murder. Jealousy, paranoia and possibly the occult add to the story's ominous atmosphere.