Did my new medications cause my sister's death, and did she forgive me for my bad treatment of her, and does she need people's prayers despite their belief that she is one of the people of Paradise and will intercede for us, and can she come to us in dreams and can we see her in the Hereafter, and what can I do to overcome my intense grief over her?

You bear no responsibility or sin for your sister's death. You must resist your grief by accepting God's decree. If your sister was of sound mind, she was accountable and will be judged. If she was mentally incapacitated, she was not accountable and is among the people of Paradise, as long as her parents were Muslims. Her intercession for you is a matter that Allah has kept exclusively within His knowledge, and your kindness towards her will not go unrewarded. Seeing her in a dream may or may not happen, and the best refuge you can seek is patience.