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. 

 -