What is the ruling on a son, dutiful to his mother, taking a portion of her pension money without her knowledge, knowing that this money is spent on his brother who uses it for bad things? And how can he repay it if it is unlawful, given that it is impossible to inform his mother about it?

The son must repent and return the money he took from his mother's pension without her permission, by any suitable means. He is not obligated to obey his parents in giving his money to his brother. He should also advise his mother, with a good approach, to spend the pension in its proper ways.