What is the ruling on one who swore to abandon an act and then returned to it, and what is required of him, knowing that he swore under duress, and does the ruling on one who was forced to swear apply to him?

Your brother's mere request for you to swear an oath does not constitute coercion unless its conditions are met. Therefore, expiation is incumbent upon you for breaking your oath without legally recognized coercion. The expiation is to feed ten poor people, or clothe them, or free a slave. If you cannot find these, then fast for three days. Since you broke your oath, your oath is dissolved, and only one expiation is required of you, even if you repeat the action thereafter.