Is it considered breaking an oath if one's brother utters the sworn-upon word once, then states that he will not say it again? And if so, is the questioner, who is 15 years old and under his mother's care, obligated to fast for three days, or clothe ten needy persons, or is he given a choice between the two?

If you speak to your brother after he has uttered the word to you, then you have broken your oath and expiation is due from you. If you do not speak to him, you have not broken your oath, and your oath remains as long as you do not speak to him. It is better that you break your oath and offer expiation for it, based on the saying of the Prophet, peace and blessings be upon him: "Whoever swears an oath and then sees something better than it, let him do that which is better and offer expiation for his oath." The expiation for an oath is: feeding ten poor persons, or clothing them, or freeing a slave. If you cannot find any of these, then it is fasting for three days. Your father is not obliged to pay the expiation for you unless he volunteers to do so.