What is the ruling on dealing with a maternal uncle who has apostatized from Islam and drinks alcohol, and what is the solution for dealing with a sister who insists on communicating with him despite the family's opposition and the mother's anger?

It is not permissible to declare a person an apostate easily; rather, it requires the fulfillment of the conditions for takfir (declaring someone an infidel), the absence of impediments, and the establishment of proof against them. The one judged an apostate is asked to repent. If they do not repent, their punishment is execution, based on the Prophet Muhammad's (peace be upon him) saying: "Whoever changes his religion, kill him." No one carries out this punishment except the ruler. It is obligatory to shun the apostate, treat them as an enemy, and not associate with them or show them affection, because apostasy is more severe than original disbelief. The ruler must execute the apostate if they do not return to Islam. The danger of your sister associating with an apostate must be clarified to her.