I'm so sorry about your sister. Dementia is always a terrible thing to watch happen to a loved one.

Something you can do that can help a little is to make a memory book. It can be as simple as a little photo album with a one-two sentence statement about her life on each page ("I like baseball. My favorite team is the Giants") ("I worked in a library") etc. Or you can go fancier and use a photo album or scrap booking album and include pictures with labels on all faces, meaningful mementos (Like a program from a wedding) and just kind of make "the story of Sister's Life). I used to do these with my patients with dementia and they usually loved them. One woman with more advanced dementia lost hers and they had to search her whole room because she really missed going through it every day. It was the first thing she'd enjoyed in years. It won't stop the dementia; it just reminds her of important things in her life.

I hope that she has a slow progression and that you are able to spend time with her. It's good she has people who love her.