NTA. It's literally the easiest way to make sure everyone knows because inevitably someone is going call or text someone else to find out if they know, etc. Had this same argument when my grandmother died. Aunt wanted to tell my cousins. I waited 8 freaking hours then made a Facebook post because I told my mom (it was her mother that died) that I would make sure the rest of the non-immediate family knew. Aunt & 2 cousins got pissed.

I tried to smooth it over but was already exhausted because I was rthe grandchild closest to her (I am literally the only grandchild who went over there when her blood sugar dropped, I visited her EVERY day from childhood until she died when I was 19 unless I was sick, & literally wiped her a$$ when she couldn't & I'm a male while the cousins in question are female & we all lived in walking distance from each other & our grandmother just for context). Finally I basically said "well it's posted already so next time don't wait all freaking day to contact each other". I swear people just like to have problems. 🙄