What should I do about my sister, who has reached thirty and is not married because of my father's refusal, and has deviated to drinking alcohol and immodest display? Should I inform my family about this, given my fear for my sister and my fear of God?

It is not permissible for a father to prevent his daughter from marrying a suitable man (Kufu'). If he does so, he is considered a guardian who impedes marriage (A'dhil), and his guardianship is revoked. The sister can take her case to the Sharia judge to have him arrange her marriage. Efforts must be made to save the sister from the path of disobedience by advising her and clarifying that these matters are grave sins (Kaba'ir) leading to loss in both worlds. If she does not respond, the family must be informed so they can intervene and prevent her from these acts of disobedience, unless doing so would result in a greater harm.