What is the religious ruling regarding a sister who refuses to annul her engagement to a married man with children, lives an unrighteous life, boycotts her family, and disposes of family property without legal right? And what is the ruling on boycotting her and causing her harm?

You must advise her, remind her of Allah, and warn her of His punishment. Her relatives should prevent her from impermissible outings, such as going out with a non-mahram (definition) man or to places of debauchery and sin. Her abandonment of her mahrams and her sister is not permissible. There is no sin upon you, O questioner, in boycotting her after advising and reminding her, but continuing the advice is more appropriate if it benefits her. If the apartment is owned by your father, you have a right to it that does not expire. If it is rented, she has no more right than you to your share of it as a rented apartment. The reference for this matter would be the Sharia (definition) court.