Is it permissible to take money from my brother after having sworn an oath not to take it, knowing that my mother wishes for me to do so, and what is the expiation for breaking an oath?

It is permissible to atone for an oath and take the money from your brother, based on the words of the Prophet, peace and blessings be upon him: "By Allah, if Allah wills, I do not take an oath and then see something else as better than it, except that I atone for my oath and do that which is better." The atonement for an oath is to feed ten poor persons, or clothe them, or free a slave. If you cannot find any of these, then it is to fast for three days.