If you mom is competent to say why she doesn't want your sister to know, then I'd respect her wishes. Your sister sounds like a real piece of work.
The one thing I'd recommend, though, is to keep this consistent. If Sis has been like this all along, then I'd consider her to be permanently out of the loop and I'd confirm that with Mom. I wouldn't agree to be a party to any game-playing.
Edit:
I've just read your other thread, and this is indeed game-playing. It's what my husband's side of the family does, and it's childish and stupid because no doubt they'll make up at some point, and you'll be made out to be the bad guy if you don't let it go as easily as they do when that day comes.
I'd tell my mom that it's up to her to notify anyone if she is or has been in the hospital. I'd also tell your sister the same thing, then remove yourself from their idiotic drama over a perceived Christmas Day insult.
Do some reading on triangulation. That seems to be what is going on here.
Here is a link that might help. Also take a quick look at the book “Boundaries” by Townsend and Cloud. You can find a lot on the internet about their work.
Also, the sister that wasn’t told about mom has two grown children that have never called their grandmother and the last time this sister called her mom was 5 months before she went into the hospital. This sister never contacted her mom to wish her a Merry Christmas and neither did her children.
The sisters do not have a good relationship. The sister that did know sent the other sister a birthday card 4 months before and the sister (not told) sent it back “refused”. Also, the mom had a cellphone with her in the hospital and said that she would call whoever she wanted to let them know.
1) First and foremost, what is the reason mom doesn't want her other child to know? Are they estranged? Or is mom one of those "oh no, don't tell anyone, I don't her to worry" pseudo-martyr types? Estrangement is one thing, but the other reason leads to all sorts of ramifications.
2) And nearly equally as important, what is the relationship between the 2 sisters? Are they close? Are they not, but the daughter "in the know" is hoping to repair a damaged relationship? If "in the know daughter" doesn't tell her sister, will it cause a rift that can never be repaired, and will that upset "in the know" daughter?
My husband wasn't told by his oldest brother when their mom got very sick to the point she almost died. Brother didn't want to "worry" DH. It almost caused my DH to cut all ties with his brother.
When my sister was pregnant with my niece, she called us one night to say she was out of emergency surgery, she had almost died due to complications with the pregnancy. I still remember the look on my mother's face - she was totally gob smacked. "I didn't call you before the surgery because I didn't want you to worry".
When my FIL had to go to the ER once, he asked us (DH & I) to not tell SIL...FIL said he would call her himself. So she could hear his voice and "not worry". Well, as it turned out, he couldn't get to his phone in the ER in a timely fashion, and when SIL got home from work, she had no idea where he was. When she called us, I had to tell her that her dad was in the ER; she was furious with me and DH for not telling her. We made up, but that also almost caused a rift in the family, and that I would have regretted greatly, because I truly love my SIL as one of my sisters.
Because if these incidents, we have a standing policy in my family to let everyone close to us know all serious medical issues or concerns as soon as we find out about them. I value these relationships too much to risk them on "I didn't want you to worry". Because being closed mouthed about medical issues also causes a different kind of worry, the "what aren't they telling me" kind - which, in my opinion, is a far worse feeling.
The one thing I'd recommend, though, is to keep this consistent. If Sis has been like this all along, then I'd consider her to be permanently out of the loop and I'd confirm that with Mom. I wouldn't agree to be a party to any game-playing.
Edit:
I've just read your other thread, and this is indeed game-playing. It's what my husband's side of the family does, and it's childish and stupid because no doubt they'll make up at some point, and you'll be made out to be the bad guy if you don't let it go as easily as they do when that day comes.
I'd tell my mom that it's up to her to notify anyone if she is or has been in the hospital. I'd also tell your sister the same thing, then remove yourself from their idiotic drama over a perceived Christmas Day insult.
Do some reading on triangulation. That seems to be what is going on here.
Here is a link that might help. Also take a quick look at the book “Boundaries” by Townsend and Cloud. You can find a lot on the internet about their work.
https://www.choosingtherapy.com/narcissistic-triangulation/#:~:text=Another%20common%20way%20that%20narcissistic,out%20the%20narcissistic%20parent%27s%20approval.
Note: I’m not saying anyone is a narcissist. Just read this and see if it feels like what is going on with the mother’s family.
https://www.agingcare.com/questions/i-am-asking-this-again-because-i-should-have-given-more-information-481060.htm
1) First and foremost, what is the reason mom doesn't want her other child to know? Are they estranged? Or is mom one of those "oh no, don't tell anyone, I don't her to worry" pseudo-martyr types? Estrangement is one thing, but the other reason leads to all sorts of ramifications.
2) And nearly equally as important, what is the relationship between the 2 sisters? Are they close? Are they not, but the daughter "in the know" is hoping to repair a damaged relationship? If "in the know daughter" doesn't tell her sister, will it cause a rift that can never be repaired, and will that upset "in the know" daughter?
My husband wasn't told by his oldest brother when their mom got very sick to the point she almost died. Brother didn't want to "worry" DH. It almost caused my DH to cut all ties with his brother.
When my sister was pregnant with my niece, she called us one night to say she was out of emergency surgery, she had almost died due to complications with the pregnancy. I still remember the look on my mother's face - she was totally gob smacked. "I didn't call you before the surgery because I didn't want you to worry".
When my FIL had to go to the ER once, he asked us (DH & I) to not tell SIL...FIL said he would call her himself. So she could hear his voice and "not worry". Well, as it turned out, he couldn't get to his phone in the ER in a timely fashion, and when SIL got home from work, she had no idea where he was. When she called us, I had to tell her that her dad was in the ER; she was furious with me and DH for not telling her. We made up, but that also almost caused a rift in the family, and that I would have regretted greatly, because I truly love my SIL as one of my sisters.
Because if these incidents, we have a standing policy in my family to let everyone close to us know all serious medical issues or concerns as soon as we find out about them. I value these relationships too much to risk them on "I didn't want you to worry". Because being closed mouthed about medical issues also causes a different kind of worry, the "what aren't they telling me" kind - which, in my opinion, is a far worse feeling.
I don't think anyone can add anything.