Fin rot and ammonia burns can take a long time to heal. Good on you for trying for so long.
The food you are using is not the best. At some point, try to get some New Life Spectrum Betta food or Omega One Betta Buffet pellets. You won't find them at Wal-Mart, but you will find them at Petco and PetSmart and many local pet stores. I realize money is tight for you now and you may not be able to do this right away. Just do it when you can. Most betta foods on the market are not good for betas -- check out the ingredients lists. If the first ingredient isn't fish (not fish meal, but some kind of whole fish), its probably junk. If the second ingredient is wheat, it's probably still junk. The pet trade really tends to go for what is cheapest when making pet food and the health of all our pets tend to suffer. Proper nutrition will really help to keep them healthy and their immune systems working well. Also, soaking pellets for 10-20 min before feeding is a good idea.
You might want to talk to your parents about your sister's fish care. What she is doing is just plain cruel, even if that is not her intention. Fish have a central nervous system and a brain just like people do, and they feel pain, boredom, stress, etc. What you sister has been doing is the equivalent of keeping a dog locked in a closet, almost never cleaning out the poop, etc. and once in a while tossing some food in there. For a fish its even worse since they *have* to swim in their own wastes. They breathe in their own toilet water. Tell your parents and your sister about this. No "pet" should have to live that way.