Is there a sin upon my brother's wife or upon me if I take the money bequeathed for marriage and benefit from it without touching the principal amount, until Allah permits me to marry?

Your mother's bequest to you is a bequest to an heir, which is prohibited by Islamic law and is not effective unless approved by the adult, mature heirs. If they approve it, then the money is yours, and there is no sin upon you for taking it before marriage. You are obligated to pay Zakat on it if its conditions are met.