Mom's going on vacation, but she doesn't know why?

The sister who financially devastated my mother's nest egg by giving it to her derelict brother, a jobless failed recovering addict, to the tune of almost 25% of her net worth threw up her arms after a five year stint of scorched earth and neglect (fiscal) and dumped it all on my doorstep abruptly. POA, Medicare, Health Care and Housing (Assisted Living) the whole enchilada!

But no problem as I am very interested in Mom's well being and took the baton in stride. But, there is always a but, now six months later that same sister wants to disturb Mom's first six months in her newest assisted living facility. (She moved Mom through 3 AL's in two years! now she lives in Florida and has now moved her from Virginia.) This is being done within the critical acclamation period for Mom. Mom is barely involved in the community. She is also very uncomfortable with new comers. She is basically a loner. But sister has the idea to take her again away from her new environment and drop her off after reorienting her for four weeks at her house in Vermont! Not a good scenario, so I believe.

Any way, as POA, I must have some right to refuse my sisters initiative? Can't I just say no? Mom isn't very able to do so herself as she is easily manipulated and convinced in her current confused early Alzheimer's status.