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