The Godsend (1980)

When a strange pregnant woman arrives at the Marlowe home and gives birth there, she vanishes soon after, leaving Kate Marlowe with the baby girl, Bonnie. The English family raises the child as one of their own, but as the years pass, their household is shaken by a series of unexplained deaths among their children. Kate’s love for Bonnie is tested as suspicion begins to settle on the girl they took in.