Is a woman's marriage invalid if her guardian was her maternal uncle, despite the presence of her brother who did not object, due to the absence of the father whom she only saw once?

The maternal uncle cannot be a wali (guardian) in marriage, as wilayah (guardianship) belongs to the ‘asabah (agnates) such as the father, grandfather, and brother. The father's right to wilayah does not lapse due to his shortcomings or absence, unless the absence is definitive, in which case the wilayah transfers to the next in line.

The aforementioned marriage was conducted without a wali, which is invalid according to the majority of scholars. If the judiciary in your country adopts the Hanafi madhhab and validates marriage without a wali, then the marriage remains valid. Otherwise, the contract is void and must be renewed in the presence of a wali if both parties desire it. A ruler's judgment validating a contract in disputed matters cannot be overturned.

Scholars have cited the saying of the Prophet, peace and blessings be upon him, "There is no marriage except with a wali," and his saying, "Any woman who marries herself without the permission of her wali, her marriage is invalid, invalid, invalid," as evidence for the invalidity of marriage without a wali. If both parties wish to continue the marriage, the contract should be renewed.