AITA for not calling my MIL's doctor for an emergency?

I have been married for 14 years and we have 5 year old triplets. My mother in law has a history of calling an ambulance and going to the hospital for things that she thinks are an emergency but are not. For example, she will call an ambulance because she has a headache and is convinced she is having a stroke. I've tried to leave my husband deal with those calls but it is hard when you get a call from your mom at 3 am saying she needs an ambulance. The hospital will not talk to you if you are not the emergency contact so I have to get out of bed and go to the hospital with our kids. A few years ago I had had enough and signed a power of attorney saying my brother is her person and not me. This has helped a lot. I no longer get calls to go to the hospital when she gets worried. I still have to take care of the kids when he has to go but I can at least stay home and sleep.<br><br><br>Last night I received a call from my husband telling me that his mom is dead. She was 85 and has COPD, diabetes, a pacemaker, and several other issues. He said he was going to be with family. I told him to take his time and the kids and I would be fine. We had to contact several people last night and this morning to let them know. I made arrangements with his aunt to take care of all the paperwork and told my husband he didn't have to worry about anything. A few hours ago, I got a very nasty text from his sister saying that I'm the Worlds Most Selfish Woman. I asked her what she was talking about and she told me that my husband is upset because I didn't call an ambulance. I was very confused and told her I didn't know he was sick and thought he was with family. She told me that she had called around 3 am and told me to call an ambulance that their mom was dead. I told her that I didn't get the call because I was asleep. She said that she hung up and called 911 because she knew I wouldn't call. I asked her why she didn't call my brother since he is the medical contact and she said "He's not family". Now my husband is calling me saying that I killed his mom because I didn't call 911. I tried to explain to him that his sister is not my emergency contact and if she wanted an ambulance she could call one. He told me that he wants a divorce because I'm an awful person. Am I really the asshole here?