If the Bride’s Wali (marriage guardian) is her brother, does he have to perform the actual contracting? Is it sufficient that he approves and witnesses the contracting process that is materially performed by her maternal uncle (considered the head of the family by the surrounding social customs)? Alternatively, would it satisfy the Rukn (integral) that the brother actually delegates the maternal uncle to undertake the contracting?
In our Madhhab, when the brother happens to be the highest figure in the sequence of marital guardians, the marriage cannot be contracted without him unless he delegates a guardian of a lesser degree like his paternal uncle to execute it. In this case, it would be sufficient.
And Allah knows best
source : http://www.lamppostproductions.com/when-the-wali-is-the-womans-brother/