What should the questioner do to atone for her sin after she contributed to an act of sorcery—which her mother-in-law claimed was for guidance—against her divorced sister-in-law, so that the latter would marry against her will?

If you knowingly gave your sister-in-law magic to drink, then you and your husband's mother are partners in the sin and harm of magic. You must sincerely repent and seek forgiveness from the one who was harmed. However, if you did not know it was magic and your mother-in-law deceived you, then there is no sin upon you between you and Allah due to the lack of intention. But you are a partner in guaranteeing the harm resulting from it. So if she dies, half the blood money is on your 'aqilah (male relatives responsible for paying blood money), and the other half is on her mother. If she suffered less harm than that, you are responsible for it, and if it reached one-third of the blood money, it is also on your 'aqilah.