First of all, what type of fish are we talking about, just so that we can look at PH level and so on
You are going to nee a test kit, at least to test for nitrite, nitrate, PH and ammonia. If any of these levels are down, we can help to give advice on how to rectify this
In a one gallon tank??? Do you have a filter or heater in there. If not, this could be why, there is no bacteria cycle in there and any food will just be rotting and will be turning to ammonia. Your sister needs to get a tank pronto!!! Even better, find a friend with a cycled tank and ask if your sisters fish can be a roomie in there until a permanent house is brought