Bismihi ta'ala

If your question is regarding who the Wali 'Amr is, and a wali amr in this case refers to the Islamic guardian, who is the father, or the father's father, then there are few scenarios:

If the girl is a virgin, and not religiously independent (rasheedah) her wali amr can be a male who her father has appointed in his will. If the father has not appointed anyone in the will, then her wali amr is a Jurist who carries all the required conditions of Ijtihad. The Mujtahid that she selects will be her authority and her guardian. This means that the Mujtahid can approve the marriage, and hence it will be legitimate. 

If she is rasheedah, and rasheedah in this Islamic context means a woman who has not been married, but is individually, socially and financially independent and she alone makes her own choices and decisions. In this case, she does not need consent from anyone.

In any of these above cases, it is crucial that family members be involved as well. For example, even though from a shar'i perspective she does not need to seek consent from her mother, or her uncles, or elders, but it is indeed morally and socially very important to do. 

And Allah knows best.