I do not believe that HM King Abdullah underestimates his younger brothers. I believe that the precedence comes with age, so from older to younger, as seen in many royal families in the Arab world. Accordingly the priority would be for Feisal, Ali, Hamzah and then Hashem. All have been regent at one time or another. Once CP Hussein comes of age, then he would probably take precedence over King Abdullah's brothers. It therefore has nothing to do with full/half brothers. King Abdullah has supported all his brothers and they are there to serve their nation.