What’s wrong with my Betta fish?

It's always hard to identify when a fish is doing poorly. But what I always recommend is making sure they have the right environment. That's always key to having a healthy fish. Once they're in the right environment, if they still don't get better, then you can look into further treatments.

Your fish needs at least a 5 gallon (19 L) tank. Think of it this way: You could live in a box if you were given food and water, but would you be happy? No. Your fish needs space.

Then, make sure you have a water heater to keep his tank a constant warm temperature. Make sure when you add new water, it is as close to the same temperature as possible. You mentioned you use drinking water - is it distilled? Because sometimes distilled water is missing some important nutrients needed by fish. If you use tap water with a water conditioner, he would be just fine.

Filtration! Trond Hansen posted a link to the Nitrogen cycle above. Fish waste and discard food cause high ammonia levels, which are extremely harmful to fish. Filtration will help with this (though you still need to do regular water changes), and it also encourages good bacteria to grow.

Finally, make sure you're testing the water for all those chemicals. This can help you identify if it is incorrect levels causing your fish's illness. Check pH, Nitrates, Nitrites, Ammonia, and water hardness. If your sister never cleaned his tank, it's likely the ammonia levels were too high.

Once you are able to transfer him (slowly! acclimate him first) to his new environment, keep a close eye on him to see if he improves.

Best of luck to you and your fish!