Am I sinful for what I did regarding my wife’s grandmother, after I tried to cure her by reciting the Qur’an and then presented her to a sheikh who gave her "Hab Al-Muluk" (a poisonous plant), whereupon her condition deteriorated and she passed away?

May God have mercy on the woman, and we ask God to expiate her sins and elevate her ranks. The fundamental principle is the acquittal of one's conscience from obligations, and certainty is not removed by doubt. Based on this, you are not blameworthy for what happened, and we hope you will be rewarded for your search for her treatment. And Allah knows best.