Will a mother be punished for her son stealing money that she used to feed her poor children, and must the one who asks the question return the money?

Your brother's theft is a reprehensible act from which he must repent if he is an adult. He is obligated to return the stolen money in both cases, whether he is an adult or not. Your mother should have returned the money, even if by stratagem, because it is impermissible to benefit from stolen money. What is now obligatory upon you is to return this money. If you do not know the exact amount, then you must give what you believe to be the amount or more. Once the money reaches its owner, what you purchased with it becomes lawful for you. The money should be returned without causing disgrace, such as depositing it into the uncle's account or delivering it to him through a trustworthy person. Sheikh Ibn Uthaymeen stated that if it is difficult to directly inform the person from whom the theft occurred, the money can be delivered to him indirectly, for example, through a friend of his, while mentioning the story of repentance.