Hey, I totally get how frustrating that must feel — especially after taking such good care of your phone for 4 years. It sounds like your Redmi 9C is really starting to show its age. The slow performance, random shutdowns, and charging issues are pretty common signs that the hardware is wearing out.
Since you've already tried a new charger and it's still acting up, the battery or motherboard might be failing. Unfortunately, phones do slow down over time, especially after several years of use and a few crashes along the way.
If your sister still needs a phone, you might consider getting the battery checked or replaced — sometimes a new battery can make a big difference. But if the phone keeps shutting down and lagging badly, it may not be worth putting more money into it. You did a great job keeping it alive this long, honestly. It might just be time to retire it fully.