Evertime my older sister comes to visit from out of state, she tries to get my mother to change the decisions my mother has made regarding her future. It includes the will and how my mother should be taken care of when she cannot live independently without help. I am the executor of her will, have durable power of attorney, healthcare proxy and joint ownership on her checking account mostly because I live the closest. My mother is 82 and competent, but somewhat vulnerable to dealing with my controlling sister. With the will, my sister wants to exclude our step sister as an heir. My sister is also trying to take control of how my mother will be taken care of when she will not be able to live indepenently without help. All this has been discussed and decided between my mother and I. Yet, my mother calls to discuss with me my sister's suggestions. My sister is the type that will help, bu only if it also benefits her. I am ready to just turn it all over to my sister if my mother doesn't stop allowing my sister to interfer. Has anyone else had similiar problems and how they handled it?
I'm not sure your mother is able to resist your sister. Have you talked to her about it in a calm mood? Not "Goddamn it, Mom, why do you always do what Susie says?" But "Gee, Susie is kind of forceful in her opinions, isn't she? What did she say to you to make you reconsider X?" Ask her if she felt guilty, or afraid, or just too weak to resist. Ask her what she would prefer to do.
Could she say, "Susie, if I do that without talking it over with Debralee, she'll be angry/hurt. She is the one I have to trust these days, because she is here all the time. Would you discuss it with her, because I don't want to get in the middle." That makes you the bad guy, but then you can be the one who faces your sister.
Maybe you can negotiate it so your sister will get a SLIGHTLY larger inheritance than the stepsister, just to shut her up. Maybe you can agree with any suggestions she makes that actually make sense, again, to shut her up. Then you can stand firm on the other stuff. If you are calm and reasonable, she might be embarrassed to pitch a fit.
I would suggest asking your sister not to do these things because they upset your mother, but I suspect that would be a waste of breath. Try not to blame your mother. She's another victim.
One last thought. Does your sister feel left out of the planning? Could the THREE of you have a discussion of what you have decided and what your sister thinks would be better? Again, she might have a good idea or two, so listen to her ideas and reasons respectfully and with an open mind.