What is required of one who killed his mentally retarded sister

Question:
Honorable Shaykh, may Allah grant you success. This questioner also says that he is not from this country. He says: I am an elderly man. I had a sister who was mentally retarded. One of the people of evil took advantage of her mental retardation and committed zina (unlawful sexual intercourse) with her. I killed her with the help of my brother. He says: This was many years ago. What is obligatory for me now that I have grown old, given that I was ignorant at the time?

Answer:
Yes, you killed a soul without right. And it was a soul close to you, one with a mahram (a relative one may not marry) relationship, so the crime is severe. But you must repent to Allah the Exalted. If she has heirs, you must pay their share of the blood money from her diyah (blood money), the diyah for intentional killing. Yes. And if they forgive you, praise be to Allah. Yes.