"If the government chose to warn travelers before they left that they would be up for the cost, then that would be fair enough"
Yes, in principle that is reasonable.
However, this situation did not just appear out of nothing but as a result of a serious and changing situation that was apparent at the time your sister left. Clearly apparent to your mother.
Those who left after the Government issued the "do not travel" advice should accept the consequences of their decision to ignore it and accept the cost of that decision.
However, the rapidity of the changes is no doubt why the Australian people are generously picking up the tab so far.
So, what would now be a suitable warning that if you return to Australia you will be quarantined for 2 weeks at your own expense ?
