I've also found appealing to authority works wonders too, as mum isn't a Trumpy type, rather an authoritarian type of conservative. "Mum, <the health authority> states <health guidance>."
Just so long as it's an institution and isn't her 50 something babychildren telling her things, it can go over well.
So, when my mom's mom died, my mom said that her mom would never listen to her, and if she started to do that, to call her on it - I have, once [and it did work]. The problem is, my mom is basically a good person so she, like too many of us, thinks the best of people, even when there is proof.
Also, she knows I get my news from the 'internet' and luckily, she doesn't trust the internet at all, even though people she does trust on the TV pull stuff off the internet. Thing is, she will listen to my husband directly, but if I invoke him as one of my sources on what's going on over in Ukraine and Russia, she won't listen to me on that front either.
'Mom, my sources are Russian and Ukrainian. I'd think they'd be right on things; more so than non-Slavic sources.' 'But the world has changed so much since I was a kid, so its changed there too, its not Soviet Russia anymore' And so it goes. And on US stuff, well, since her mom died, she's suddenly going more conservative and I blame CNBC [she watches it constantly for stock market stuff, but also, 'they can't lie because if it messes up the stock market....' so she watches their news too.