Would you believe, in the movie Two weeks with love(1950) they were sisters Patti and Melba Robinson
and in the movie Athena (1954) they were sisters Athena and Minerva Mulvain.
A lot of people don't know that Anthony Perkins had a slight but very appealing singing voice and had something of a recording career. I've spent quite a few hours on youtube searching out his musical videos.
Here he is singing "Summertime Love" from his Broadway show Greenwillow.
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And "Takin' a Chance on Love."
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Though my favorite Perkins musical performance is the TV musical Evening Primrose, music and lyrics by Stephen Sondheim.