Am I doing the right thing?

Very long story short (as I can, anyway). My mom has not been diagnosed with dementia - yet. I have had an EMT, and an ER Nurse, and Dr ask me if she has it. "Not diagnosed" is my response. Memory issues, self care issues, time, can't work electronics (or a landline phone) she has all the symptoms, until she is in front of strangers (witnessed by my fiancé and adult daughter). So about a year and a half ago, took a fall - ended up in rehab for some PT, came home. 2 months later, took her on a trip across the country, with me managing her meds, etc. and she did very well. 4 months later, another fall - er trip ended up coming home, returning a couple of days later violently ill, admitted with Covid - for a third hospital trip 2 days after being released, she had bronchitis and pneumonia. Siblings are not local, so I do updates via messenger. My sister has not called nor spoken to my mother in at minimum 1 year. When she called me before all the hospital trips, I asked her if she wanted to talk to Mom. No is the only response I got, and then no more calls. Not Mother's Day - not her birthday - not Christmas. Nothing. I have never had a relationship with my sister, nor do I care to. I tolerate and am very amicable with her for my mother's sake. My brother, also did not call on her birthday, nor Christmas, but called Easter (first call since late summer) I am absolutely livid with the both of them, but that is MY issue, not hers.Mom told her hair stylist she talks to her children on the phone, but not mine (when it is her grandchildren that actually DO CALL) which breaks my heart, but it is what it is. Here is where it gets wonky. Sat evening I get a call from my brother (voicemail, as I was busy with mom and don't answer calls when I am) that my sister was "resting comfortably" in the hospital after having what appeared to be a mini stroke. OK, fine I have been informed. It's late, I don't respond - so I get another call the next day, only he told me she was now home. I informed him that after careful thought - I was NOT telling my mother about my sister. At this point, I can NOT travel with her, so she would basically just sit and feel useless, and I don't think the stress would be good for her. to which my brother so very graciously gave his approval to. Yesterday, I missed a call from my sister, the voice mail saying it's her and she is in the hospital and she'll try later. Well, I have also decided I am not answering. She basically cut off contact a year ago, so why should I? Does she want my sympathy?

I feel very bad for not telling my mother, but I truly feel it would do more harm.

So very sorry for the length, but everything is so convoluted it seems to take forever to explain.