I'm all for rescuing sick fish--but be aware that when you simply buy a sick fish from the pet store, that another fish will simply take its place next week. When you buy sick fish from a pet store, you are encouraging a system that is not held accountable for the health and quality of their fish, and though you have helped one fish, you may just as soon as damned the next one. I urge people not to make sympathy purchases in this manner.
If you see a sick fish that you want to save, get the manager involved, tell them what is wrong with the fish, and that you will take it as long as you can get the fish for free, or at least at cost value. If you can't, then leave. It's hard to do this but if you keep giving the store money, their policies will never change.
I hope your fish recovers and that your family understands later--but if they don't, don't let it get to you. Tell them they should be happy you're helping fish instead of doing drugs or eating glue or something.