President Lincoln had relatives (on his wife's side I believe) who fought for the South." Confederate Cavalry Commander JEB Stuart's father-in-law fought for the Union. Other examples include Union General John Gibbon who had brothers who fought for the south. As they parted ways, Armistead said something along the lines of "may God strike me dead if I ever lift a hand against you." When the war started, Armistead and Hancock were serving together in California. Close friends Winfield Scott Hancock and Lewis Armistead fought on opposite sides with General Armistead receiving his mortal wound during Pickett's Charge in front of General Hancock's troops at Gettysburg. Henry Wentz, who also fought for the Confederates, stormed Union lines near his boyhood home just south of Gettysburg. Wesley Culp for whose family Gettysburg's Culp's Hill was named, fought for the South and died during the battle. Rudolph Schwarz who fought for the Union on day 1 found his brother among Confederate prisoners. "There are so many just during the Battle of Gettysburg. I would love to know of any specific cases where this is true, such as best friends forced to fight against each other." "This war is described as 'brother against brother'.
An e-mail I received in March of 2004 asked this question: One of the reasons I remain fascinated with the American Civil War concerns the idea of brother fighting against brother.