I know exactly what you mean. As I said my g-g-grandfather and his three brothers were in the 3rd TN Mounted Infantry USA. I have another g-g-grandfather, who with two brothers were in the 11 George Infantry CSA. They had three brothers with the 52 GA Infantry CSA who were at Vicksburg and the oldest brother of the family was with the 6 GA Calvary CSA.
Anyway one of the daughters of my g-g-grandfather[Morrow][USA] married a younger brother of my g-g-grandfather [Garrett] [CSA]. You know they had some fights..lol.
By the by, my family roots are in Cherokee County NC.From what I understand that area was basicaly lawless during the Civil War. Don't know that you could say it was a Civil War in a general sense as meaning brother against brother and father against son. I'm sure that most people from New York were total strangers to those from Florida. But it was definitley kin against kin in Western NC and Eastern TN..and also Northwest Alabama.