One brother is in the Army when he marries & still has time to serve. The chances that he won't return alive are pretty high & so his brother agrees to remain single until he has left the Army in order to be able to care for his widow & children. (in the meantime, the wife would, presumably, take care of her brother in law's household) An alternative scenario could be that the man not in the Army is illegitimate while the one in the Army is legitimate or the man not in the Army is from a poorer branch of the same family & agrees to do this in return for some (probably financial) assistance of some kind. In view of his age, the man staying at home might already be a widower with children.
Any offers ?