So, my sister stonewalled me today. She said if I spend the $60 on a phone that our mother can't dial out from, she will just disconnect it and take it and not give it back to me. Oh, and she's a mother of 3 and she's 47 years old.
I give up. Going to have to just keep my cellphone ringer off now, all the time.
I did find some online caregiver forums for adult children like myself. It's a relief to read their stories so I know I'm not alone -- some of their stories share the asshole siblings they have too. *They* being the other adult children caregivers on this forum I found.