Is it just me?

Is it just me?

My parents "learned" on me, the oldest child.  They were stricter and forced me to be independent.  In retrospect, they did me a huge favor.  I am an independent adult, now age 55.  My sister, 5 years younger, has been clingy and dependent all her life.  "She's the baby." has always been the excuse for extending her allowance into adulthood, allowing her to lie, cheat on her report card, break the household rules, etc. 

 Now twice divorced, my sis can't support herself and writes hot checks, gets pay day loans from Cash Stores at 600% interest, gambles, and still lies.  She is in poor health, constantly broke, cannot afford food and medicine because she is constantly trying to stay out of jail from her hot checks etc.  Before my father's death, my brother and I learned that my sis had borrowed over $20,000 from my father and step mother and had not repaid any of it.  When my father died, my sis received her 1/3 of the estate.

 Now mother is in poor health and has been in the hospital.  Her estate has been on her mind and sis's fate is on her mind.  She asked me for my brother and I to be sure and give sis her fair share of her estate and not cheat her out of it, "like we did" with my father's estate.  I replied to her that we had not cheated anyone, that everyone had received their full one-third of the inheritance.  My sister had lied to my mother.

I have so many issues with this.  First, my mother knows my sister lies, and yet has apparently believed this untruth for 11 years, never asking me or my brother about it.  On the other hand, my brother and I do not lie and she has believed the worst about us.  That hurts beyond belief.  

Mom is considering converting her garage into an apartment for sis, so they can take care of each other.  I was in favor of this until I learned that sis has been lying about me.  Now I feel very betrayed.  I have helped sis over the years as much as I can, despite my belief that she is a poor money manager.  

 I would like some opinions as to why my mother might be setting my sis and I up to be against each other.  I should comment that my mother is showing early signs of dementia, and was sick.  But, when I asked her directly "is this something you THINK we did, or did sis TELL you we cheated her?'  she responded Sis told her we cheated her.  

 Mom and her own sister have feuded for more than 20 years and have just reconciled.  She claims she wants my sis and I to be close.  If so, why is she setting us up to mistrust each other?  Mother even commented to me that sis spends more time with them, and I seem not to want to be around them as much. (hello, I'm married and even though I'm retired, I am doing 3 part-time freelance jobs, and I have friends I enjoy being with).. sis is single and Mom is her only friend. I talk to Mom by phone almost every day, and see her at least once a week.

I have to confess that I have also considered that I have thought that my mother could be devious enough to "work" us where we cut off sis, then mom would think she has grounds to cut us out of her will and leave everything to sis.  Mom is a very devious person.  When Mom talks about sis it's always "Poor ___."  that's her mindset.  I have been successful and worked and stayed in my marriage.  Sis works hard at 2 jobs, but has never tried to get ahead or be promoted because she is fearful that she can't do the job, or she doesn't want the extra responsibility. 

someone please help me define my fears and help me figure out how I should handle this situation.  I think it would choke me to do nothing and pretend everything is lovely. 

 Am I offbase to think of my mother as toxic? she feels that way to me.  I'd appreciate any feedback.

Thanks.