It's unclear who is having the surgery (mom or sister) and what their physical limitations would be.

Comfortable slippers, a nice housecoat or robe (if breast cancer surgery, there's special robes for that), cozy blanket

Some games: https://www.amazon.com/Codenames-Duet-Player-Word-Deduction/dp/B072J234ZF/, https://www.amazon.com/Scrimish-Strategy-Card-Game-Portable/dp/B018SUEAE0/, https://www.amazon.com/SET-Enterprises-4001-Five-Crowns/dp/B00000IV35/

Madlibs: https://www.amazon.com/Family-Tree-Libs-Roger-Price/dp/0843116439/

Coloring Books (if knee replacement, check out https://www.amazon.com/Color-Laugh-Through-Replacement-Recovery/dp/1518841740/), Paint by Numbers, a good book, 1-month Netflix subscription (or similar) if they'll be resting up on the couch a lot

These aren't board games, but fun card games, which is fun because they're nice and portable if you're ever out somewhere board and/or having to wait around.

• Love Letter is a fun card game that can be played by 2-4 players. It's an easy game to get the hang of and is probably more fun with more players but works just fine with two. You play trying to "secretly" with the affections of the princess. The cards have special abilities that may enable you to connive and/or cunningly take advantage of your opponent(s). (Amazon link)
• Jaipur is a also a 2 player (only 2 player) card game, which is kind of more resource trading/collecting based -- good if you're familiar with Catan. (Amazon Link)
• Codenames Duet may be up your alley as well. I really love Codenames and haven't played much of Codenames Duet, but rules are essentially similar. You try to "group" words based on commonality. This works great for partners/SOs because you normally can predict pretty well how the other may think. You can use this to your advantage or to sabotage, hahaha. (Amazon Link)