What should a sister do if she was sexually abused by her own brother as a child for many years? Both brother and sister are now married. Is the sister guilty of backbiting because she told her family what her brother did? Is it okay for the sister to seek help with her mental state because of what happened to her, e.g. doctors, etc.? Please help me with this issue.

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 your brother had sexual intercourse with you while you were young or after reaching the age of puberty but without your consent, then you are not sinful. However, if anything of this incest had taken place after you reached the age of puberty without being forced then you have to repent for this sin. If you sincerely repent then the Mercy of Allaah is vast.

You should know that if a Muslim is afflicted by committing some sins, it is not permissible for him/her to inform others about it without a necessity, as Allaah likes a person to conceal his/her sins. The Prophet said: “All my nation is pardoned [by Allaah for what they commit] except those who sin openly or disclose their sins to the people." [Al-Bukhari and Muslim]

What would help you rid and forget the bad psychological effects of this act is to return to Allaah and seek His Help and be keen to be pious and fear Him. It is permissible for you to consult a psychologist if you need to do so, but you should choose one who is experienced and trustworthy. Opt for a female psychologist rather than a male, and do not inform the doctor of anything except according to the necessity. Moreover, you should not inform the doctor about the name of the person who committed the act with you, as this is more serious than backbiting him.

Lastly, we draw your attention to an important matter which is that if you notice that your brother is looking at you with lust, then wear Hijab in front of him and do not let him stay with you in seclusion. You should treat him as a non-Mahram. 

Allaah Knows best.