Is the marriage contract valid if the agent in it is the sister's husband, given the absence of the sick father who is estranged from his family?

The guardian of the woman must be present at the marriage ceremony or appoint someone to act on his behalf. The contract is not valid without his permission. As for the man, he may appoint someone to act on his behalf in his marriage, such as his sister's husband, even if his father is alive. Agency in marriage is permissible. However, if your brother is an alcoholic, his testimony to the marriage is not valid, because the uprightness of a person who commits a major sin is nullified. One witness is not sufficient; two upright witnesses must be present for the contract. The majority of scholars hold that witnessing is obligatory at the time of the contract, and if two upright witnesses are not present, the contract is null and void and must be renewed.