Hello Sarah and

Welcome to Fish Lore. Ilovee has given you some good information. I would like to add that your goldfish really needs a larger space and with filtration. Moving your goldie to your sisters tank may be a good idea if she has other goldfish to keep it with. It's not suggested that you mix goldfish with tropical fish as goldies are cold water fish and tropical fish require warm water.
Keep doing your daily water changes and add some Prime, if you can find it in your area, to detox the ammonia for 24 hours until it's time for the next
water change. The Prime will also help the fishes natural slime coating which helps to fight off disease and infections.
Best wishes for a full recovery!
Ken
