I participated with my mother-in-law in casting a magic spell upon my sister-in-law as I gave her the potion to drink so that she would get married. What should I do to atone for this sin?

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 you knew that you were giving your sister-in-law a magic potion, you are as sinful as your mother-in-law, and you are both responsible for anything that may happen to your sister-in-law, subsequent to the spell. It could be that she goes insane. In this case, you must  repent sincerely to Allaah The Almighty and ask your sister-in-law  to forgive you.

However, if you did not know that the drink was a magic potion and your mother-in-law deceived you into it, you are not  considered sinful because you did not intend to do that.

Nevertheless, you would be a partner in the liability of any bad consequences that may result. For example, if this woman died, your family should pay half of her blood money, because you were involved, even if it was done by mistake. Your mother-in-law  is to pay the other half of the blood money because she was the major cause. This is because the one who did the act and the one who caused it are both held responsible for it.

If the resulting harm called for paying less than one third of the blood money, you would be liable to it, but if it called for one third or more, your ‘Aaqilah (paternal male relatives) would be liable to pay the blood money.

Allaah Knows best.