I need some easy game advices that can be played with one hand.

Bejeweled 3, Peggle, Plants vs zombies, Peggle, most point and click games (you can't go wrong with Lucasarts), Warsim: realm of Aslona, NEO Scavenger, The caribbean sail, hidden object games, most digital board games, Omen Exitio:Plague.

I will recommend a mouse: the Logitech G600. It has a configurable 12-button pad.

Or the Razer Tartarus v2 Gaming Keypad, depending on which arm she broke.

As someone who has broken twelve (yep, not a typo) bones, tell your sister that I know this sucks. But, I can also attest that she'll heal up just fine. I've clearly done it a bunch of times. :D

Time to become a video game nerd...and maybe upgrade that machine when money isn't tight from paying for surgery. I hope that she's able to power through the COVID and get her corrective surgery soon. In the meantime, ibuprofen and ice packs are her friend.

Some of steam's golf games are pretty fun if you're into that sort of thing. I really enjoy "Golf it!"

You could maybe use Steam streaming (or Discord, or whatever) to play escape room/puzzle games together. I don't know how many actual multiplayer ones there are.

I suggest Grim Dawn - game itself may be a bit complicated and complex, depending on playstyle, but controls are easy. You don't even have to click that much - just hold mouse button and point. Pure dzen. And yet its pretty involving, addictive and rewarding.

Don't Starve Together, but you have to set up server with mods to make it (relatively) easy. Namely, Automatic Health Adjust.

Divinity: Original Sin 1 & 2

Wild Terra Online - was pretty good casual timekiller before it went F2P.

All those games have co-op, options to be low-demanding on hardware, difficulty adjustments and playable with mouse only.

BTW, thanks for your comment about GTA V at my profile.
Later i missed my won PLAYERUNKNOWN'S BATTLEGROUNDS at freegamelottery - not claimed T_T.

I hope she gets well soon! XCOM Enemy Unknown/Within can be played entirely with mouse and it has multiplayer

If you have a licensed copy of Windows 7 on that PC I'd definitely suggest getting the free upgrade to Windows 10. Some games require Windows 10 to run them, like Fallout Guys. That's a really fun game you and your sister would probably enjoy.

I hope your sister has a speedy recover from her broken arm as well as covid.

Little Inferno

Among Us, I play with one hand, just my mouse hand(have even switched hands to see what it would be like and was still fairly easy), it's cheap, and multiplayer up to 10. So you can have all your friends join you both. As far as Carona goes, the recovery rate is extremely high for people under the age of 80. I too have broken my elbow, it's no fun. I wish her a speedy recovery.

Sims 4. Sorry I didn't notice that made weeks ago, I hope get her quick recovery.