Is the marriage contract considered valid if the wife's guardian is not mentioned in the contract, despite the consent of all adult parties and the presence of the wife's brother instead of her guardian (her paternal uncle or maternal uncle)?

For a marriage to be valid, it must be contracted by the guardian (wali) or his proxy, based on the Prophet's (peace be upon him) saying: "(There is) no marriage except with a guardian and two just witnesses." The woman's guardian is her father, then her paternal grandfather, then her son (if she has one), then her full brother, then her half-brother (from the father's side), then their sons, then the paternal uncles, then their sons, then the father's paternal uncles, then the ruler.

If the girl does not have a grandfather, her guardian is her full brother. Any of her adult brothers can validly marry her off. Among the pillars of marriage are offer (ijab) and acceptance (qabul). The offer comes from the guardian or his proxy, and the acceptance comes from the groom or his proxy. The mere signing of the marriage contract by the woman in the presence of her guardian is not sufficient; rather, her guardian or his proxy must undertake the contracting of the marriage. So the brother says: "I marry you to my sister [so-and-so]..." and you say: "I accept." The contract must be redone with this formulation to be valid.