Well, she has dropped it for now with me, and also cancelled her family's planned trip to visit my parents next month, and lied about the reason, to boot, in our group text. She is unaware that my mom told me about their recent argument. So, hopefully this is the end of it. I have been sharing medical updates with my sisters and they have kept things civil by thanking me for sharing them.
I am not unwilling to hash things out with her, but I am not willing to allow her to make things about her RIGHT NOW, when we need to be focused on helping my mom and dad through this situation. Now is simply not the time.