What kind of living arrangement is best for sister in mid 60's with moderate Alzheimer's?

Assisted Living sounds appropriate. Or some kind of group home/foster care arrangement.

If she can live alone and only needs general supervision.. I would say assisted living. I would find one that has a memory care unit so that she can be moved to a higher level of care when needed.

I'm so sorry.. mid 60's is so very young. Maybe find a facility that at least has some younger people closer to her age.. if that is possible.