Is it permissible to send money to a jailed brother for him to buy food, despite the mother's prohibition of it?

There is no harm in sending money to your brother without your mother's knowledge. You are not obligated to obey your mother in refraining from sending him money, so long as he needs it for permissible food and does not use it for disobedience. However, if your mother's prohibition is for a preponderant benefit, such as the prohibition leading to the child's righteousness, and sending money preventing his righteousness, then obedience to your mother becomes obligatory at that point.