Mom and my sister who is POA. How do I get my sister to make a decision about Mom's care before she leaves ?

My Mom has did mean things to my sister and I and she's doesn't trust anyone other than my other sister, who is POA. She is not around to see what all Mom does and it makes for a tense relationship with her because she just looks at us like this doesn't happen to her when we tell her what Mom has done. Mom just got out of the hospital for a major operation and in a few months my sister will be leaving for the winter and hasn't made a decision on what we can do with Mom. I've had medical problems(cancer-2 times) and I'm still facing another operation and my sister that helps with Mom works 40/50 hrs a week and her health is pretty much gone as well.
How do I get my sister to make a decision about Mom before she leaves for the winter? I help as much as I can but it's really hard. Thanks!