Is it permissible to demand the money that the father took and refrained from buying the promised animal for sacrifice (udhiyah), and is the father sinful for that? What is the advice on dealing with the situation where the father and mother gave the Umrah money to the younger brother instead of performing Umrah?

It is not permissible for a father to dispose of the money for which his son appointed him as an agent, in a manner other than what was authorized. The son is entitled to demand it back from him. However, the son is advised to forgo demanding the money from his father, seeking reward from Allah Almighty, out of filial piety towards his father, and maintaining kinship with his brother. If he wishes to advise his father, it should be done with gentleness, kindness, and consideration for the obligatory filial piety.