Is it permissible for a woman to be married off by her sister's husband, who is her guardian, with the presence and authorization of her brothers, knowing that her father is deceased?

We have clarified the order of guardians in Fatwa (definition) no. 67866. If a woman's guardianship transfers to her brothers, there is no objection to them delegating someone else, such as their sister's husband or another person. Scholars have explicitly stated the permissibility and validity of a guardian delegating someone to act on their behalf in conducting the marriage contract. Some even deemed it commendable if the proxy is a virtuous person. Ibn Al-Majishun said: "There is no harm in the one marrying off delegating the woman's guardian to a righteous or noble man to conduct the marriage contract, and this was practiced in the past."