It depends. Not knowing your mom, this is difficult to answer. Does she become easily confused and then become anxious? Does she hallucinate? Is she paranoid about changes in her routine like so many people with dementia are? What happens if you take her out to lunch or for anything else for just a few hours? That experience would be magnified by her being gone for a few days. Even if she’s been there before, chances are she might not remember. Think about how she has reacted in the past to changes in her routine. If you need her to go because you are getting away for a while, maybe having your sister come to your house instead might be a better idea.
Whose idea is it for Mum to go to your sisters?
Where is Mum living now?
How far is it from her home to your sister's.
How will she get there?
If it is to give you respite, what about switching homes with your sister for a few days? Less upheaval for Mum that way.
Or if you sister just wants to visit, have her stay in a nearby hotel and come to your house to visit Mum as her routine allows.