All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
An irrevocable power of attorney is a new method introduced by modern civil laws as a means of authenticating sales.
In legal terms, it is divided into two sections:
First:
An irrevocable power of attorney against a charge. This is a reliable legal method of authentication and doesn`t violate the rules of Sharia because the selling is done in a valid manner; therefore, the method of authentication makes no difference, and the authorized enjoys the right of transferring the ownership of the sold property to whomever he likes, even after the death of the authorizer.
Second:
An irrevocable power of attorney against no charge. In other words, this is a grant/donation , and it ends upon the death of the authorizer, and the authorized has no right to benefit from it after his death (authorizer).
Accordingly, all the property-whose ownership wasn`t transferred during the father`s lifetime- is an estate that must be divided amongst all his eligible heirs, including your half - brother. The grants or donations given by the father to some of his children are valid although Islam recommends justice in this regard. So, it is better for those who have received more, while not being in need, to give it back to be divided as an estate. Ibn Hibbaan narrated in his Sahih that the Prophet (PBUH) said(what means): “ Observe justice when bestowing gifts and grants upon your children, so that they observe justice in their dutifulness to you “And Allah knows best.