What is the appropriate action to take when dealing with a sister who disobeys and mistreats her parents, and insists that she is in the right despite her mother's attempts at reconciliation, and what is the ruling on a brother boycotting her in this case?

First: Supplicate abundantly for her guidance and righteousness, based on the Almighty's saying: "And your Lord says, 'Call upon Me; I will respond to you.'"

Second: Advise her with gentleness and kindness, reminding her of Allah and instilling fear of parental disobedience. You may seek assistance from people of virtue. Abandonment (hajar) is permissible if it does not increase her stubbornness.

Third: Obey the mother in showing kindness to the sister, for that is an act of obedience in goodness and may be a cause for her righteousness, based on the Almighty's saying: "Repel [evil] with that which is best."

Fourth: Forgive whoever wronged her, even if she was in the right, and appreciate the virtue of forgiveness. For the mother has a right to dutifulness and kindness, even if she wrongs.