Oh op I'm so sorry. Being a bereaved parent is truly the hardest thing, but with the most supportive community 💐
Two of my children have died, and I would have felt the same way as you as well.
Your child dying takes away any bit of control you have, it's something so incomprehensible to most people, so you want to have a grasp on something in your life, and your sister has taken away your right to tell your sad news your own way.
I will say that lots of us are so angry at the situation in general when our child died that we find something tangible to be angry about. For me, when my son died, the thing I got angry about was totally unjustified, but it almost made me feel better at that time to be able to direct anger somewhere. When my daughter died the thing I got angry about was justified tbh.
From this outside perspective I would say you have every right to be angry at your sister, and I would seriously consider cutting her right off for this, don't be hasty though, take some time and make the right decision for you.
This path, unfortunately, lasts a lifetime, so there no hurry to work through your feelings and decide anything right away. Take each second one at a time at the moment and focus on getting yourself through the coming days and weeks in the most gentle way possible for you. Everything else can wait 💐