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BattleBlock Theater is a pretty good one if you both like puzzle based platformers. It also has single but coop levels are different and you need other person's help to finish them. Also it is pretty funny and family friendly.

I played the whole Trine series as Couch coop with a friend.
Also Moving Out and Overcooked 1&2 is fun for two.

Shiftlings is a very fun two player game

Can you please provide some info on what genres your mother prefer?
Because I'd recommend X-Morph right away as it was extremely fun for me, but not sure if your mother (or you) is going to like it.

Lovers in a dangerous spacetime

Some ideas for you maybe:
All Trine
Unrailed
Hacktag
39 days to mars
Alpaca Ball Allstars

Keep Talking and Nobody Explodes is quite a fun coop game

If your mom doesn't mind being a backseat gamer (e.g. watch you play, provide commentary etc) then Girlfriend Reviews offers some pretty good suggestions

https://store.steampowered.com/app/252110/Lovers_in_a_Dangerous_Spacetime
https://store.steampowered.com/app/471550/Nine_Parchments/
https://store.steampowered.com/app/880940/Pummel_Party/
https://store.steampowered.com/app/506500/Party_Panic/
https://store.steampowered.com/app/610180/The_Jackbox_Party_Pack_4/
https://store.steampowered.com/app/477160/Human_Fall_Flat/
https://store.steampowered.com/app/413150/Stardew_Valley/ (not sure)

https://store.steampowered.com/app/269030/The_Yawhg/

https://store.steampowered.com/app/794800/ClueCluedo_The_Classic_Mystery_Game/ (this is not possible on one PC of course but maybe with remote play on another PC or a smartphone/tablet)
https://store.steampowered.com/app/431240/Golf_With_Your_Friends/
https://store.steampowered.com/app/809880/Degrees_of_Separation/
https://store.steampowered.com/app/396710/The_Adventure_Pals/
https://store.steampowered.com/app/286160/Tabletop_Simulator/

Magicka 1 and Magicka 2

Overcooked is quite nice. Portal 2 if you guys/glas like puzzles.

I'm personally not too fond of Football but this hidden gem here is just soo adorable.

Pixel Cup Soccer 17

It can be played on one computer with 2 controllers or if I remember correctly also with controller + keyboard.

I got it from IndieGala ages ago when they still had hidden gems like this or Has-Been Heroes in their bundles once in blue moon. Fair warning though it has only been bundled once. Apparently the developer(s) know it's too good to waste it on Bundles.

A Way Out is pretty good

TOWERFALL!!
battle block theater
almost any lego game
i play a lot of jackbox games with my family
Monaco
portal 2
screen cheat
thief town
crawl
tiny brains
chariot
castle crashers
brawlhalla- meh freesmashbros
audio surf if your into music
Awesomenauts free 2d moba, super basic

Stardew Valley is nice, but requires a second device.
Overcooked!

Love Letter might be a decent other game?

binding of isaac rebirth?
edit : but modded so that you can actually play as 2 different characters instead of a character and a co op baby

"A Way out" for something different?
You need two players to actually play it. One owns the game, the other gets a restricted version to play.
2 PCs needed however.

All of the Chicken Invader games (of which you have quite a few) are pretty good, simple, silly and with co-op.
Also all the Worms games are really good for playing on a single computer.

Farm Together
Best to buy when it goes on sale... played over 200 hours with my kids!

Brothers - A tale of two sons

Never Alone
Unravel Two
LEGO series
Biped
SpongeBob SquarePants: Battle for Bikini Bottom - Rehydrated

All the Lego games are pretty good for local co-op play.
Also Hidden-Object games are fun to play from the couch. Even if one person has the controller. Both can search and find the items.