Then you charge them money and do as they ask. Or they can go ask someone else to do it for free, maybe that friend who recommended the software that doesn't run on Linux, given that they know better than you...
If you want to throw a wedge in between you and your family over this that's fine, but you did break it by preventing them from doing what they wanted with their computer.
Very quickly most will get the idea that the computer is broken and you need to come and fix it because you were the one that broke it.