It was an old B&W horror about identical quint boys who are about 10yo. 4 of them were adopted out to good families. But at 10 they start getting terrorized and murdered. Turns out it was by the 5th baby. The grandmother comes forward and explains that #5 killed her young daughter so she threw the baby into the ocean to die.
Can you name this movie? My friend told me she has looked for the name for years!
I've never heard of your film but I think the story has to be based on the Dionne Quints.
http://flashbak.com/the-dionne-quintuplets-story-in-photos-kidnap-freak-shows-and-hollywood-glitz-9468/
Thought it would be easy to locate but alas I came up empty handed.
Looked at twin and multiple movies, nada.
(The list isn't complete, not at all. The Other 1972 isn't even on it.)
A Complete List of Twins & Multiples in Movies
http://www.twinstuff.com/multiples-in-the-movies/
Marking this post so I can get notified if anyone locates your movie. Wishing you the best of luck.
Enjoy the song while others search for your flick.
The Parent Trap (1961)　Let' Get together