Is the husband's authorization of his brother a general power of attorney that includes buying and selling, without leaving anything to prove the children's rights, a deprivation of their rights? And is the wife's saving from her husband's money without his knowledge, with the aim of preserving her children's money, forbidden by Islamic law?

The summary of the fatwa (definition) is: It is permissible for a father to appoint an agent or make a will concerning his mature children, and he may dispose of their wealth. No one has the right to object to his actions. It is not permissible for the wife to take anything from her husband's money due to his agency for his brother. Her right is preserved in his estate after his death.