"I think that Sissy Spacek is probably a bad example. In Austria and Germany ""Sissy"" or ""Sisi"" is also short for Elisabeth, try the a search for sissy in 
 de.wikipedia.org. So given that Ms. Spacek middle name is Elizabeth, her Mother is of German descent and her father is of Czech descent (which once was a part of the Austrian empire) I would guess that this is the true origin of her Nickname. 

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