Is my sister's marriage invalid if her guardian was her maternal uncle while her brother was present, and what should she do if it is invalid?

The maternal uncle is not a brother. If he falsely claims brotherhood, the contract is void, and her guardian should handle her marriage, or the ruler if she has no guardian from her lineage.

If her brother was present at the contract and appointed the maternal uncle or remained silent and his consent was evident through circumstantial evidence, then the marriage is valid.

Silence does not indicate consent unless accompanied by circumstantial evidence that points to it. If such evidence is absent, the contract is invalid and must be renewed in the presence of a guardian, two just witnesses, and a dowry.