My mother has I believe stage 7 Alzheimers and is taking 100mg of Trazadone and a antidepressant, this allows her to sleep and get rest, where before this she would ask for cars they have been gone for years. My sister seems to think that we are over medicating her and this is the reason she can't speak and doesn't eat. I've read articles on the Alzheimers and her symptoms are that of stage 7 and I am her main caregiver.
You are the primary caregiver and coping with moms needs I presume; sister should mind her own business. If she would like to take care of mom for a few weeks, then tell her you would welcome her help and the respite.
If you know when sis is coming, you could consider delaying moms dose to have mom more alert when sister is visiting if that is the concern. Ask sister to come and have mealtime with mom to help spur moms appetite although I think this has everything to do with the stage of the disease and is normal for the progression -- but maybe sister needs to witness for herself.
Do what you think is best to keep mom calm and comfortable. Talk to your dr about any concerns.