No matter what I do, Mom ends up sleeping in her chair or on the floor. What do I do?

Put a pillow under her head and cover her with a blanket

I quite often sleep on the floor when my back is crook. It is actually quite comfortable then, more comfortable than my mattress. Make sure that she has blankets she can grab, perhaps a pillow, and think about putting down a rug that gives a bit of padding. Check for drafts, which can be common at floor level. It's nothing to worry about if she wants to sleep there.

Emily, I see from your profile that your Mom has arthritis, so maybe sitting up in a chair is more comfortable for her. As for sleeping on the floor, that is unusual, maybe with her dementia she has fear of falling out of bed.

Compare the bed that she uses when your Mom is visiting with the siblings. See what is the difference.

Could there be a light source that bothers her. Or if the room is too dark, install a couple of night lights. Oh, what about Mom's pillow? Check to see how that is different at the sibling's house.

Are you using a scented detergent and/or softener when washing the sheets and pillow cases. I know I have to re-wash my bedding if sig other accidentally uses the scented detergent/softener which he likes. My nose gets all stuffed up.

Also, is Mom eating any snacks before going to bed, but not eating snacks at the sibling's house.

One has to play detective to figure out what is happening. Hope you solve this mystery soon.

Years ago, we had a problem getting our baby to sleep through the night in his crib at a new house. He would wake about every 20 minutes. We finally realized the the HVAC vent was blowing right at him and it would wake him when the air came on. We put on a plastic diverter and it helped him sleep through the night.

I hope your solution is as easy to fix as mine was!

The floor is a concern because of the possibility of pressure sores. The chair is not much of a problem if a large lounger, I think. If not a large lounger, then that may be a great thing to get for her. It often does put the body into a comfortable, back supported, head raised for easy breathing, and can sometime be recommended for some conditions such as COPD, Pulmonary issues and gastric reflux.