What make is your sisters printer?
they may be right, although not all cheap ink the same. It's like everything, there are good one and bad ones.
My printers are a Canon Pixma and I have 3 Epson's. In all cases, after the ones that came with the machine were empty I moved on to clones and then to refilling them myself. One of my Epson's takes four cartridges and if I had to only use Epson genuine refills, I wouldn't be able to afford to run it. Depending on where you buy them they can cost between £50 - £80 each!
As for the Canon, I've just bought a CIS (Continuous Ink System) This is by far the cheapest and most economical way to go. Unfortunately they don't make a CIS for my Epson.