Family Plot (1976) on Netflix

Wealthy and aged Julia Rainbird wants to bequeath her vast fortune to her only presumed living heir, her illegitimate nephew, who was given away as a baby and as such whose whereabouts and identity she does not know. Rather than go to a private investigator to locate him, a P.I. who would use public means to do his work, Julia instead enlists the more discreet services of her psychic, Madame Blanche, to locate her nephew, with a payment of $10,000 upon delivery. What Julia is unaware of is that Madame Blanche - really Blanche Tyler - is a charlatan, who uses basic information of her targets often uncovered by her actor/taxi driving boyfriend, George Lumley, to perpetrate the scam. Blanche and George feel this $10,000 is their meal ticket, despite having only scant leads as to the identity and whereabouts of the nephew. Their initial leads indicate that the nephew may in fact be dead... or perhaps not. In reality, the nephew is Arthur Adamson, a jeweler, who, along with his secret associate/girlfriend Fran, have amassed their own small fortune through the theft of gems, largely through kidnapping the gem owners, with the said gems being the ransom paid. Arthur grew up as Edward Shoebridge, the adopted son of friends of the Rainbird's chauffeur. Along with another associate, Edward killed his parents in a house fire twenty-five years ago, and faked his own death in the process, although he had never been declared legally dead since there was no conclusive evidence of such. He assumed his current identity after the fire. When Arthur learns that these strangers, George and Blanche, are trying to locate Edward Shoebridge but not knowing the reason, Arthur and Fran believe it's because of their latest kidnapping, and thus decide they need to do whatever, including murder, to keep George and Blanche from finding "Eddie".