In the Name of Allaah…
In the West, it is not too uncommon to find a woman who disputes with her father about a potential spouse, who then goes to an Islamic center to get another walee (legal representative for the marriage) appointed for her, so she can get married without the father’s permission.
Sometimes, another relative is brought in to take the father’s place, and other times a walee is requested to be appointed from outside of her family.
Let’s be perfectly clear here – Playing games with wilaayah (the right of being the walee) can lead to the marriage contract being invalid, even if it has stamps and seals, with many witnesses and corroborators!
Legislated Ranks of Wilaayah in Marriages
Let us take a moment to be clear on the legislated ranks of wilaayah in a marriage contract. The following are the order of ranks, each of them being sound-minded adult men of the same religion:
- Grandfather (from the father’s side)
- Great Grandfather (from the father’s side)
- Great Grandson
- Oldest Full Brother
- Next Oldest Full Brother (and so on…)
- Oldest Half-Brother (from the Father’s side)
- Next Oldest Half-Brother (from the Father’s side, and so on…) 
- Oldest son of a Full Brother (Nephew)
- Next Oldest Son of Full Brother (Nephew, and so on…)
- Oldest Son of a Half-Brother from the Father’s side (Nephew)
- Next Oldest Son of a Half Brother from the Father’s side (Nephew, and so on…)
- Grandson of Full Brother (oldest first)
- Grandson of Half-Brother from the Father’s side (oldest first)
- Oldest Paternal Uncle
- Next Oldest Paternal Uncle (and so on…)
- Oldest Son of Paternal Uncle (Cousin) 
- Next Oldest Son of Paternal Uncle (Cousin, and so on…)
- Oldest Great (Paternal) Uncle
- Next Oldest Great (Paternal) Uncle (and so on…) 
Important: Each of the above mentioned walees has the right to appoint someone to act on his behalf. So in this case, if the father appoints the brother to act as the walee, then he is the official and legally accepted walee, even in the presence of the grandfather, since the brother is the father’s chosen trustee and takes his place.
After these relatives have been exhausted, then a woman may seek to appoint a walee from outside her family through the Muslim authorities.
Any violation of this order of rank – for example, excluding the father and getting married with the brother as the walee – leads to the contract being invalid.
A recognized judge may determine that a man is unfit to be the walee, due to special reasons related to his inability, like drunkedness, oppression, irreligiousness, or prolonged absence or refusal to marry her to anyone, etc., In these cases, the judge could appoint the next in rank as the walee.
This is an issue determined by qadhaa (an official court judgment), not something decided by a phone call or an uninvestigated fatwaa.
In some cases, a Muslim woman is from a non-Muslim family, and thus none of her male relatives qualify due to a missing condition of wilaayah, ittifaaq ad-Deen, a common religion. In such a case, she seeks to appoint a walee other than her male family members.
A Verdict from the Permanent Committee in Saudi Arabia
A question was posed to the Permanent Committee of Scholars in Saudi Arabia, headed at that time by Shaykh Ibn Baaz (d.1420, may Allaah have mercy on him) :
Sometimes a woman’s brother is appointed as the walee in place of her father or grandfather, appointed (by the rightfully ranking walee) and agreed upon. Is the marriage valid in such a case, when a lesser-ranking walee marries off the woman with the availability of a higher-ranking walee, so long as he agrees? What is the ruling (in this case)?
When a lesser-ranking walee marries off a woman with the availability of a higher-ranking walee without any valid excuse or without him authorizing it (the change of walees), such a marriage contract is invalid, and thus the resulting marriage remains legally ineffective. This is because such a person has no right to be the walee for the woman, so long as someone more deserving is available, meaning the higher-ranking relative.
However, the next highest ranking walee may take the place of the rightful walee who relinquishes his right to wilaayah. Also, a marriage contract is also valid and the resulting marriage is legally effective when the rightful walee entrusts someone to represent him as the walee. This is because it was his right that he gave up and passed on to someone else who takes his place (legally).
Therefore, it is permissible for the brother to marry off his sister, so long as the higher-ranking walee (the father, grandfather, etc.) has appointed him and placed him in charge of the marriage. 
And Allaah knows best.
Written by: Moosaa Richardson
 Ibn Qudaamah mentions that there is no differing amongst the scholars that the brother is next in rank after the father, grandfather, son, and grandson. There is some differing about the ranks of the full brothers and half-brothers on the father’s side, and those who come next in rank as well. (al-Mughnee, 9/358)
 The scholars differ over when a cousin wants to marry his ward to himself. Some scholars allowed him to be the walee and the groom. Others said he must allow the Muslim authorities to appoint a walee. Others said he may appoint someone himself. See: al-Mughnee (9/373-375).
 al-Mughnee (9/355-370)
 Fataawee al-Lajnat ad-Daa’imah (18/174-175), signed by Shaykh Bakr Aboo Zayd (member), Shaykh Saalih al-Fowzaan (member), Shaykh ‘Abdullaah Ghudayyaan (member), Shaykh ‘Abdul-‘Azeez Aal ash-Shaykh (vice-chairman), Shaykh ‘Abdul-‘Azeez ibn ‘Abdillaah ibn Baaz (chairman).