Is what the deceased brother left for his mother considered a gift, knowing that he deposited the amount into her account during his lifetime and allowed her to dispose of it when needed?

A brother's وصية (bequest/will) that his brother should give his mother everything he owns after his death is not valid if it exceeds one-third of the estate or if it is for an heir, unless the other heirs agree. As for what the mother took from her son's money during his lifetime with his permission, there is no harm in it. However, what remains of it after his death is added to his estate. If all the adult heirs of the deceased brother agree to give the mother the entire inheritance, there is no harm in that; otherwise, she is only entitled to her legal share.