Salaams... JazakhAllah for your question
If all your close male relatives are non-Muslims, then a kaafir cannot be the wali (guardian) of a Muslim.
Ibn Qudaamah said: With regard to a kaafir, he cannot be the wali of a Muslim in any situation, according to scholarly consensus.
Ibn al-Mundhir said: Those from whom we acquired knowledge are unanimously agreed on that.
Imam Ahmad said: We have heard that ‘Ali allowed a marriage done by a brother, but he rejected a marriage done by a father who was a Christian. Al-Mughni, 7/356.
The question here is: what should a Muslim woman who does not have a Muslim wali do?
The answer is:
A Muslim who is in a position of authority or status should do the marriage contract for her, such as the head of an Islamic centre, the imam of a mosque, or a scholar. If she cannot find anyone like this, then she should appoint a Muslim man of good character to do the marriage contract for her.
Shaykh al-Islam said: In the case of a woman who does not have a wali among her relatives, if there is in her locality a representative of the ruler or the chief of the village, or a leader who is obeyed, then he can do the marriage contract for her with her permission. (32/35).
Ibn Qudaamah said:
If a woman does not have a wali or a ruler, then there is a report narrated from Ahmad which indicates that a man of good character may do the marriage contract for her with her permission. (7/352).
Your brother in Islam