My awesome "adopted" mother died (on Easter) of a drug overdose. Based on the autopsy, they could not tell if it was intentional or accidental. Her husband had died 6 months prior and she was depressed so it clearly could have been intentional. However, her daughter was getting married in a couple of months and she was really looking forward to the wedding. She had gone to an Easter service and said she was going to start going back to church regularly. She had drug addiction issues in the past which further complicated things. We'll never know if it was done on purpose or a terrible accident.
Point being, I don't go around saying she killed herself. I just say it was a prescription drug overdose. There is no proof either way and most people don't want to hear the whole story.
Like me, I think you might have to accept that you'll never know the truth. If she was at fault, your Nsis likely will never admit to doing anything wrong simply because that's how Ns are. You will never get an admission of guilt for anything. On the other hand, it really may have been an accident. Doctors can't always predict when a patient will have a bad reaction to a drug. Bottom line, you really don't know what happened.
You're really focused on the fact that your Nsis didn't answer the phone which in itself is not proof of killing anyone. They were having a fight and she wouldn't answer the phone which seems normal especially when dealing with stubborn Ns.
You're assuming your mother would still be alive if your sister would have just answered the phone. But would anything really have changed? Would she believe Nmom's complaints or just think she was being dramatic and ignore her? Throughout my childhood I had to listen to my Nmom whine about how she thought she had a stroke or a heart attack. Her blood pressure was high ONE TIME at a dr visit and for months she acted like she was dying. I can easily picture your sister ignoring your mom's complaints.
Also, you even said your mom refused to go to the hospital. Would she have gone if Nsis told her to? Your sister could not have saved her life just by talking to her on the phone. I really think you need to forget about the whole not answering the phone thing. Your mom needed to go to the hospital and she didn't. Ultimately, it's your mom's fault and no one else's.
I also don't understand why you want to go around telling people your sister "killed" your mother. That's just tmi. No one needs to know that - especially if your sister was not convicted. Even if there was an admission of guilt or there was a ton of damaging evidence and she got off on a technicality, I still don't think I'd be blurting it out to anyone that she was killed. Your family problems are no one else's business.
It seems like you're harboring a lot of hate towards your sister (normal - not blaming you for that) but you just need to start letting it go. Remove the hate from the equation and just tell people your mom died from a medication issue. No one needs to know the grisly details. Yeah the Mexican meds are fishy and she made a deal to avoid criminal charges, but deals like that don't happen with hard evidence. Your sister avoided criminal charges because there was no proof to convict. Without the conviction, it's inaccurate to say she truly killed anyone. I know it sucks. Lots of hugs!