Is my giving money to my brother to help him complete the building of his house, without my father's knowledge and my refusal to authorize my father to manage my account, considered an act of disobedience that warrants my father's anger towards me?

There is no harm in financially assisting a brother, even if the father prevents it. Hiding this assistance from the father is better to avoid his anger. It is not obligatory to obey the father's request to create a power of attorney if it will cause harm. One should advise the brothers to fear Allah and avoid causing the father's anger.