Is it permissible for an alcohol drinker to be an agent and guardian over his ill mother, who suffers from "memory loss," and manage her money?

A mother suffering from mental instability necessitates her interdiction to safeguard her wealth. A guardian should be appointed for her, based on the Almighty's saying: "And if the one who owes the debt is incompetent or weak or unable to dictate himself, then let his guardian dictate justly." Drinking alcohol is a great sin, but فسق (transgression/immorality) does not prevent one from undertaking the agency over wealth, unless the agent is extravagant or oppressive. The interdiction and agency must be carried out by a judicial ruling from the Sharia (definition) courts.