Joshua Chamberlain, commander of the 20th Maine, and his younger brother, Tom, were not unique in that many close relatives served together in units, particularly in the state-supplied volunteer units in which recruiting and enlistment occurred on a very local level. The 20th Maine, for example, had several sets of brothers, uncles, cousins, and this was typical.
On a statistical basis during the Civil War, lieutenants such as Tom Chamberlain had a relatively high mortality rate, certainly higher than Lieutenant Colonels and Colonels like Joshua Chamberlain. Because he knew, as the Army of the Potomac approached Gettysburg, that the battle was going to be especially deadly for everyone (because of the stakes involved), Joshua Chamberlain was particularly concerned that both he and Tom would be essentially in the same place at the same time and therefore the chances that one or both could be wounded or killed increased exponentially.
Chamberlain's concern for his and Tom's fate was dramatically increased by the position held by the 20th Maine--the end of the Federal line and one of the most inviting targets for the Confederates. The unit's vulnerability and the fact that the Chamberlains would naturally be near each other during the fighting led the older brother to be seriously worried, not so much about dying, but about what their deaths would mean for their parents.