Greedy and rude sister!

My mom was just diagnosed with early stage Alzheimer's. My sister who lives in another state comes home to visit about 6 times a year or so. Since finding out about our mothers conditions she has been saying things to my mom such as, I want this or I want that. Meaning moms things. Mom is a little annoyed and upset about this. Saying, "It makes me feel as if she is waiting around for me to die so she can have my stuff." Our father is taking care of her and he doesn't like this at all. Mom ask us to please not say anything to my sister however. But I feel very annoyed myself about her behavior and don't know if I should go against my mothers wishes and have a heart to heart with my greedy acting sister before she comes home for Christmas and if I do and she tells mom, which she probably would, mom will be hurt by me going against her wishes. It's really been making me sick to my stomach thinking of her acting this way towards our mother and my father will never go against what mom wants. Though I wish he would.