What is the ruling of the religion regarding a sister’s estrangement from her family due to her marriage to a married man and her seizure of her father’s apartment, and what is the ruling of the religion regarding shunning her and not attempting reconciliation with her?

You should advise the girl and remind her of Allah. Her relatives should prevent her from engaging in prohibited outings if they are able. If they are not able, they should continue advising her and instilling in her fear of Allah's punishment. It is not permissible to sever ties with her mahrams (unmarriageable kin) and her sister. You bear no sin for shunning her after offering advice, unless continued advice would benefit her. As for the apartment, if it belongs to your father, you have a right to it. If it is rented, she is not more entitled than you to your share. The reference for this matter is the Sharia court.