Look up the 35th Battalion VA Cavalry versus the Loudoun Rangers:

When General Robert E. Lee's army moved north as part of the Antietam campaign, White's Comanches were suddenly back in force in Loudoun County. The Rangers were sleeping in the Waterford Baptist Church when they were attacked by White's men after midnight on August 27, 1862. Surrounded, the Rangers defended their position in the brick church until almost every man was wounded and ammunition was running low. When they surrendered, it was to relatives and to boys with whom they had gone to school. One of White's men, William Snoots, loudly insisted on the right to kill his prisoner, and it took several of his fellow Confederates to force him to accept the rules of civilized warfare. The prisoner was Loudoun Ranger Charles Snoots, his brother.

Brother on brother was common in those two units, and the captains in charge of C company of the 35th VA, and B company of the Rangers were close cousins.