May Allah (SWT) guide us all to the right path always (Ameen)? I would like to ask if a mother's brother can be a Wali when the girl has no father and no one from her father's side and she have only her mother's brother, can he do her marriage contract as her Wali? Can he agree or refuse for the girl's marriage proposal without enough reason? If he can't be her Wali can she make her marriage contract in a court with the Qadhi or Shaikh as her Wali? Is her uncle's presence needed at the court? Reply soon, as this matter is quite urgent and we want to follow and apply the Qur'an and the prophet's (SAW) Sunnah.

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The maternal uncle (mother's brother) is not considered a legal guardian for his niece so he can not prevent her from marrying. Imam Ibn Qudamah wrote in his book al-Mughni that 'other than Asaba (the blood relatives from the father's side) no relative has the right of guardianship upon a woman such as the brother from mother's side, maternal uncle, mother's uncle, maternal grandfather and the like. This is reported from Imam Ahmad. And also reported from Imam Shafi and Abu Hanifah according to their one statement'.
If she does not have any such relatives then the Muslim ruler or the one who is appointed from a Muslim ruler becomes the guardian of a girl. If there is neither a Muslim ruler nor a guardian in this case any responsible person from the Muslim community will be her guardian whether he is maternal uncle or someone else. Please read the Fatwa: 83338.
Allah knows best.