What is the ruling on shunning a brother who is characterized by abandoning prayer, disobeying parents, backbiting, harsh treatment of his siblings, in addition to physically and emotionally harming his sister and looking at her with lust, especially since the sister feels comfortable not speaking to him and has performed Istikhara regarding this matter?

Neglecting prayer, disobeying parents, looking at forbidden women (mahram (definition)) with lust, and backbiting are all major sins. It is obligatory to advise a brother who commits these sins and remind him of Allah and His punishment. If he does not accept advice from his family, then one can seek the help of someone he respects so that he might listen to them. Islamic tapes or booklets discussing these topics can also be placed in his room or car. If he suffers from a mental illness, he should be presented to specialists, and righteous people should recite (Qur'an) over him. One must veil themselves from him if he looks at mahram women with lust or touches them. There is no harm in avoiding him to guard against his evil, without a complete قطعيعة (severance of ties), unless there is a benefit in boycotting him that would deter him from his actions, in which case boycotting him is permissible. For anyone who openly commits major sins, it is permissible to punish them by boycotting if there is an overriding benefit in it for them.