Any ideas to help my sister establish boundaries with our mother, who is living as an independent in assisted living facility?

Thanks in advance for any suggestions.

My mother is 84. She has had health issues (high blood pressure, high cholesterol, small strokes, heart attack, two bypass surgeries) for over 30 years. She is also a non-compliant patient and as a parent, she was controlling and occasionally abusive. At least once a year, she has medical "emergencies" that require a visit to the ER to be admitted for observation and tests for a few days. Something that, in the past, the entire family would come running and sit with her. 30+ years of that....and more often than not, nothing being found wrong...well, none of us really come running anymore. This past year, her medical 'emergencies' have increased but coincide with stressful/life changing decisions such as, moving into assisted living and closing on the house. She's obsessed with her blood pressure, taking it multiple times a day. I am one of three girls and the only one that lives out-of-state. With small children and a husband who travels, I can't drop everything for these 'emergencies'. Both of my sisters live in the same town with my mom. One has boundaries well-established that she is rarely called for assistance. The other, the one I'm concerned about, is emotionally exhausted and drops everything she has going on to run to mom's aid...basically, she and her family are 24/7 on-call for mom and she, understandably, resents it but can't figure a way out of it. The facility mom is living in....there really is NO reason for her to be calling either of my sisters for anything she needs. I just got off the phone with my sister, who is on her way over to sit with mom in her apartment. She cancelled her own appointment to run over and be with mom. Mom calls her to let her know what her latest blood pressure reading is...for attention, we imagine. Actually, she mentioned she had an appointment and mom asked how long it would be. When told, my mom's reply was "Oh." To which my sister said "I suppose I could cancel it" and mom's reply was "I wish you would, please." Big age difference between my sisters and I, the only reason I share that is because we're at different stages in our own lives and not only does the one sister get called but so does her husband and adult children.

Our father passed away 3 years ago and he catered to mom. There is no replacement for him.

As far as friends of my mom's, most have either quit calling or it is very infrequent. If she has not heard from someone for a while, she does not reach out.

I won't deny that I'm very angry with our mother because I tend to see what she is doing, as being abusive, manipulative to my sister. There is no reason that one elderly woman needs 9 adults to wait on her, hand and foot...especially when she is living in a facility that can take care of everything. Mom says she can't take care of herself but will not relinquish control of anything.

Mom will later apologize for disrupting everyone's lives. To this particular sister, she will shed tears....yes, the push/pull thing.

Any suggestions on how I can help/support my sister in establishing boundaries that she can feel good about? We have a plan for her to take a vacation, that I come down to be physically be 'there', so she can take a break without feeling guilt but it isn't for a few months (Summer when school is out).

I understand that this stage of life mom is in, has fears and anxieties all their own. There is no lack of compassion or understanding on that (although this post doesn't really reflect that). Her fears and anxieties shouldn't become all-consuming demands from my sister and her family...right?

Once again, I am grateful to you all for any suggestions to help and support my sister with this.