Is the informal marriage of a sister, with an authorization from the man to his lawyer, and in the presence of the husband's brothers and the wife's sister, Islamically valid, especially given the absence of a guardian and the presence of the woman's adult children?

Most scholars stipulate the guardian (wali) for the validity of the marriage, so it is not permissible for a woman to marry herself. This is the fatwa (definition). Contrary to the madhhab (definition) of Abu Hanifa, who holds the view that a mature woman may marry herself. If a mature woman marries without a guardian, in imitation of Abu Hanifa's madhhab, then her marriage is valid. If the closest guardian prevents the woman from marrying, guardianship transfers to the next in line, or the matter is referred to a judge. The fatwa is that marriage without a guardian is invalid, and its rectification is through offer and acceptance from the guardian and the husband in the presence of two witnesses. The woman's guardian, in order of precedence, is: her father, then her paternal grandfather, then her son, then her brother, then their sons, then paternal uncles. If the sons refuse to marry her off, then the guardians who come after them may marry her off. If all guardians refuse, the judge marries her off. It is recommended to document the marriage contract in the courts to preserve rights.