^ I agree. How large is the tank you had them in? Fancy goldfish need a minimum of a 20 gallon tank with 10 gallons added on for every fancy goldfish added, as adults. Obviously juvies can be kept in smaller tanks but honestly not by much.
What is your water change schedule?
How many gallons is the tank and is it filtered?
Is the tank cycled?
Are your sisters goldfish common (single tailed) or fancy (two tails)?
Get the goldfish out of the 2.5 gallon. Go out and get a 10gal or larger storage container from WalMart or anywhere else that sells them, usually less than $20 for something like that. Make THAT your hospital tank. It's substantially cheaper than a glass tank, lighter than glass, and less breakable. If you keep him in the 2.5gal he may suffer severe ammonia poisoning by next weekend.
Please gently inform your sister about goldfish care and their needs. Warn her that by keeping them in that 2.5 gallon, she will kill
them. I can link you to tons of goldfish resources and web sites if you'd like.