What is the ruling on a brother hitting his sister in her husband's absence, and what is the ruling on her disowning him and abandoning him?

The brother has no authority to discipline his married sister, as this is the responsibility of her husband or father. A brother hitting his sister is an act of injustice and disobedience to their mother if it is due to her defending her. It is permissible to abandon the brother and prevent him from entering the house until he mends his ways and the harm ceases, for a beautiful abandonment is better than harmful association when there is fear of harm. There is no value in the sister disowning her brother, for brotherhood is established by lineage. The sister should not respond to her brother's injustice by severing ties of kinship, but rather by advising and praying for him, and she may abandon him until he stops his injustice. Maintaining kinship ties is an act of obedience to Allah and not a price for good treatment from a relative, for the one who truly maintains ties is he who connects with his kin when they sever ties.