Why doesn't Chamberlain want his brother to be in the army? Why is he bothered by having his brother serving with him in The Killer Angels?

A Maine man through and through, Colonel Joshua Chamberlain's stand at Little Round Top saved the Union army from disaster at the decisive battle of Gettysburg. With him in the 20th Maine regiment was his brother, Lieutenant Tom Chamberlain. Older brother Joshua often worried about Tom, his "mother's favorite," and how the family back home would react if either should be killed. He remembered an earlier time when Tom was lost in the woods on a cold winter night. A search for him was fruitless, with "Mother crying," but Tom survived the night and "came back himself, a grinning kid... never once afraid."

While under fire, Col. Chamberlain saw his brother "come up, whirling through smoke, saw a rip in his coat, thought: no good to have a brother here. Weakens a man." Later, the colonel attempted to point out a weak area of defense for Tom to reinforce, but he could not do it.

... not my own brother, but Tom understood, hopped across to the vacant place and plugged it with his body... a mortal exposed boy.

The next day, Col. Chamberlain regretted using his brother to "plug a hole... as if he were expendable... as you move a chess piece." On the final day, Col. Chamberlain made a mental note to "send Tom to the rear" to spare him from the final Rebel onslaught." However, Tom remained with his brother.

Even in the most heated moments, Joshua showed concern for his brother(s). As historian Geoffrey C. Ward describes:

"As Chamberlain and his two brothers, Tom and John, rode abreast together toward the hill, a Confederate shell narrowly missed them. 'Boys,' the Colonel said, 'another such shot might make it hard for Mother. Tom, go to the rear...' "

Joshua Chamberlain regretted his brother serving with him because it weakened him as a commander knowing that he might be responsible for his brother's death in battle and the repercussions it would have back home. However, his own belief in the Union cause was so strong that he was ultimately willing to risk his brother's life--the brother that was his mother's favorite and whom he loved so dearly.