Sorry for your loss. Ich is a nasty parasite, and it can be quite hard to treat with sensitive fish in the tank.
I've used Methylene Blue for Ich several times and I highly recommend it, I keep scaleless fish like a BGK and several cats, so it's a must have med for my cupboard as it's the only thing you can use with scaleless and copper sensitive fish. It will work, but it takes a couple of weeks. Keep treating for a week after the last signs of Ich have disappeared, add a little salt and wack the temp up to 30 degrees. Remove anything that will stain, add a few cheap terracotta pots for the fish so they still feel comfortable.
Take something good from your sisters mistake and use this as an opportunity to teach her and help her learn from this. Don't let other people feed your fish without teaching them the correct way to do it first, but I do agree with everyone on this, it's not the food that has caused it, although it probably didn't help the rapid decline as it made the water quality go down.
Again, sorry for your loss. Never nice. I really feel for you.