Back to the family mess: The ball is now in their court. I just couldn't give up my mother without a fight, especially not when the consequences of doing so would mean an institution or paid caregivers for her when the time comes that she is unable to remain safely at home on her own.
A friend suggested some time back that I try to get my mother to go with me to see some sort of mediator or therapist. A little while back, I took her advice. My first step was to contact the pastoral council at my childhood church, where my mother and older sister still are members. I got back a nice note from someone saying she didn't know who to ask but would check around for me.
Several days later, I got a longer email from her that included a list of several resources and a note that said she'd gone all the way up to the bishop to get advice.
I contacted the first resource, the one with the note that that person was the one recommended by the bishop. She sent back an email that she had forwarded my information to one of the therapists in her organization and that I should be hearing from someone.
Sure enough, then I got an email from a family therapist, asking me to give him a call. I did and left a message, he called me back, and we talked for a bit. He can help, he will see as many of my family as I can get to go, and the cost will be only what I can afford (and I get to decide what I can afford).
I sent emails to my mother, brother, older sister (the one I'm having the trouble with), my SoCal sister, and our out-of-state sister, explaining all of that and asking Mom, my brother (because he's also local), and my older sister if they would go to see this therapist with me so we could get these issues aired out and resolved.
The only response so far has been from the out-of-state sister; she was wonderfully supportive and looking forward to what happens next.