Sixties horror/suspense film

[dmdavies]

Does anybody know the name of a film I remember watching in my childhood (gave me nightmares). The plot concerned a child who witnesses her insane mother killing her father with the knife intended to cut the cake on the child's birthday. Years later the child, now a grown woman, is woken in the night to be led by another woman to view a re-enactment of the murder scene. This happens several times. The woman's legal guardian then pays her a visit, with his wife, who turns out to be the nocturnal visitor. The younger woman, driven mad by the visits in the night, kills her guardian's wife, is then committed to a lunatic asylum. Her guardian inherits her legacy. The young woman's friends smell a rat and discover that the nocturnal visitor was in fact another lady, disguised as the guardian's wife. This lady and the guardian have engineered the whole thing and they proceed to live happily ever after. Except that the young woman's friends then devise a plan which leads to the guardian's new wife suspecting him of plotting her murder, and she then kills him. The young woman is released from the lunatic asylum, regains her inheritance and they all live happily ever after. If you are still with me, do you know the name of this film?!