All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Allaah The Almighty Empowered those who have attained the legal age to dispose of their properties. Allaah The Almighty Gave fathers a natural disposition to be keen on preserving the benefit of their children, and He made their handling of their children’s properties legally acceptable.
If a father authorizes another person to preserve his properties during his life time, or appoints another person as a guardian over his children after his death, no one is permitted to object to him, or to accuse him of foolishness or to make it a pretext for unlawfully handling his property without his knowledge even if he does so out of good intention or for achieving benefits. That is because the person who attained the legal age knows his benefits well.
The wife’s fear over her rights because of the power of attorney her husband made to his brother is unjustified. Her husband is responsible for her rights as long as he is alive. When he dies, the general power of attorney becomes revocable. At this time she has the right to claim her rights of the husband's estate without any objection on the part of the attorney-in-fact, the guardian or others.
Consequently, the wife knows that her husband's handling of affairs is acceptable. She is not permitted to take anything from his properties, regardless of her intention, under the pretext that he appointed his brother as an attorney-in-fact or as a guardian.
Allaah Knows best.