Is it permissible for us to boycott a father who drinks alcohol, commits adultery, mistreats his children, does not enjoin them to pray, does not pray himself, and commits immoral acts, until he repents sincerely? And what should we do to make him abandon these habits? And is it permissible for a sister to live with her married sister and her husband in this situation?

This father does not pray, drinks intoxicants, and promotes evil deeds. This is a great calamity, for Allah Almighty has commanded us to protect ourselves and our families from the Fire, and the Prophet, peace be upon him, warned against neglecting those under one's care. It is not permissible to sever ties with this parent. Rather, one must treat him with kindness and gentleness, show concern and love for him, and not obey him in sin. If money pleases him, it is permissible to give it, provided it is not used for unlawful purposes. One must suppress anger, visit him, and serve him to revive the feelings of fatherhood within him, following the example of (And if they strive to make you associate with Me that of which you have no knowledge, do not obey them but accompany them in [this] world with kindness). The sister should be cautious when staying with him, and if she does not feel safe, she should seek a safe place.