My parents had one for free, and so did DH and I (pre pregnancy). In NSW at least they were offering this vaccine free to all parents / grandparents / carers / day care workers etc as there was big outbreak in Northern NSW. It is a nasty thing for babies...

My parents did it off the cuff, like your PIL, (mum works with kids) so I mentioned to to my PIL, just really brought it up in conversation, and I was not concerned either way if they got it, but I think they did. I think it cant hurt to mention it to close family... But like everything, if you are aware there is a potential risk (like visiting a sick person) then you can take precautions.