Does the sister sin if she treats her brother as a stranger and avoids speaking to him, limiting herself to greeting him with "salaam" when they enter the house, this being due to his previous harassment of her, his refusal to repent, his not praying, and his watching forbidden content?

We advise you to continue gently advising your brother and to instill in him fear of forbidden acts and of neglecting religious obligations, especially prayer. You should also seek help from tapes and books and direct him to sermons. Be cautious in your interactions with him by maintaining modesty and avoiding seclusion. Strive to supplicate for his guidance. If he does not respond, then there is no objection to shunning him.