My sister has early dementia, precipitated by severe dehydration. In the hospital they found she had been taking multiple doses of otc sleeping pills, so the doctor took her off all pills. She lives alone and can't keep track. Now she says she hasn't slept at all since her pills were taken away. All she wants is her sleeping pills back. She can get quite abusive in her pursuit of her pills. How do we sort all this out? Her doctor said she could take 5mg melatonin, but she says it does nothing for her.
In your sister's case, I can see that it would be genuinely concerning that she has been not taking pills as directed. How will that be managed going forward? Where is she now? Is the plan for her to continue to live alone? As dementia progresses it almost always becomes impossible for the person to live alone. Then what?
How long ago was she taken off her pills?
What kind of doctor treated her in the hospital? Has she seen a geriatric psychiatrist and/or a behavioral neurologist?
My heart goes out to you and your sister. Sleep problems can be a huge challenge in dementia. She will need a specialist to help sort this out, preferably one who is very experienced with dementia patients. She may ultimately need some medication to deal with sleep issues, but under medical supervision and not on a DIY basis at the drugstore.
Your profile says you are taking care of your husband. I am sorry that you are worried about your sister, too. Does some other family member have primary responsibility for her, such as POA? That is a huge extra job on top of caring for a sick husband.
I wish you well.