I know you already made a decision OP, but here is another perspective if you are still reading.

Over the course of his life since graduating college, my father cut off his mother, sister, brother, and me, his daughter. His cousin tells me he did the same with extended family when he was younger as well.

I was close to my father, but when he married for a third time he was pretty done with me. Now it’s no contact. It is devastating. He’s in his 80s and I know his wife and step-daughter won’t contact me. If my loving adult child told me, it would be infinitely better than finding out alone on the internet or by happenstance.

Yes, you tell so you don't have to carry this burden any further. Or, no, if her response to you as the messenger is more than you can handle, or you feel like she has put you in this role that you want to reject.

My sister is estranged from my mother who is nearing her end. I imagine that someone will inform her if she dies, but I wonder if I'll want to if nobody does. I'm kind of not wanting to be the messenger anymore because it makes my sister upset with me, and she can deal with the consequences of her actions (not being in touch) on her own. You don't get to both not check in and then feel entitled to information. We'll see what actually goes down.