Is the marriage contract for her sister, conducted by the younger brother who is thirty years old, without the knowledge of the elder brother who is fifty years old, valid—considering that their father has passed away—or is the contract only valid with the elder brother as the guardian?

For a marriage to be valid, it must be contracted by the woman's guardian (wali) or his agent, in the presence of two Muslim witnesses. If the woman's father and paternal grandfather have passed away, and she has no adult son, then the guardianship passes to her full brothers. If the guardians are of equal degree, such as full brothers, it is recommended that the one who is most knowledgeable and religious, then the most pious, then the eldest among them, perform the marriage. However, if the less preferable guardian marries her with her consent, the marriage is valid, and the remaining guardians have no right to object. If the woman designates one of the guardians, that guardianship becomes specific to him, and her marriage is valid.