What is the ruling regarding a woman who does not spend on her children, and gives money to her brother, who it became clear buys alcohol with it?

The basic principle is that the financial support for young children who have no wealth is the responsibility of the father. If the father dies or becomes insolvent, and the mother is financially capable, then the financial support becomes her obligation. She would be sinful if she neglects this duty, based on the Prophet's (peace be upon him) saying: "It is sufficient sin for a man to neglect those whom he is responsible to feed." As for the mother helping her brother, it is permissible as long as she does not know that he will use the money for a sinful act.