A brother beats his sister and abuses her and no one stops him
31-3-2014 | IslamWeb
Question:
If a sister is being beaten and physically abused by her brother and no one in the family or society does anything to stop it and she cannot approach the local police, authorities or scholars, what should she do? Fight back and get beaten more or try to remain patient and what would help her keep her sanity?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
If this brother harms his sister and beats her unjustly, then this is a great evil. She should advise him and frighten him of Allaah The Almighty and remind him that Allaah The Almighty is more capable over him than he is over her. She should seek the help of Allaah The Almighty to repel his harm off of her and supplicate with the following supplication: Abu Moosa Al-Ash’ari 
narrated that when the Prophet feared a man or a people, he would say: "O Allaah, we ask You to face them and we seek refuge in You from their evil.” [Ahmad and Abu Dawood] Resorting to Allaah The Almighty and seeking His help would preserve her sanity; Allaah The Almighty says (what means): {Those who have believed and whose hearts are assured by the remembrance of Allaah. Unquestionably by the remembrance of Allaah hearts are assured.”}[Quran 13:28]
Along with this, she should use legitimate material means among which is to seek the help of some righteous people whom her brother respects, and it is acceptable to threaten him to take the matter to the authorities as this may deter him. However, it is not wise that she deals with him in the same manner and beats him as this would result in greater harm. If she can find a safe place with a trusted relative, a pious sister or trustworthy Islamic Center, then she may stay there.
On the other hand, preventing this brother from injustice and defending the oppressed is an Islamic requirement. Anas narrated that the Prophet said: "Support your brother whether he is the oppressor or the oppressed.” They said: “O, Messenger of Allaah, we support him if he is oppressed, but how could we support him if he is the oppressor?” He said: ''By stopping him," i.e. by preventing him from being an oppressor." [Al-Bukhari and Muslim]
This type of support falls within enjoining good and forbidding evil and the conditions for it to be an obligation are the same that must be met in enjoining good and forbidding evil, like being able to do so and being safe from considerable harm. An-Nawawi 
said: “Supporting the oppressed is among the communal obligations, and this falls within enjoining good and forbidding evil, but the order to do it is for whoever is able to do it while he does not fear any harm.”
Therefore, if someone is able to support the oppressed and he does not fear any definite or probable harm by doing so, then he is obliged to do so, otherwise he would be sinful for neglecting to do so.
Allaah Knows best.