Dawn, you're doing a great job.
You've got the right attitude. When someone has been a JW for that long they will maybe only respond to very gentle reasoning.
My mum's been a Witness for about 40 years. When I first left we had some massive arguments but over the years I've learnt to bite my tongue, not get angry when the "truth" is mentioned and will even ask about how certain brothers are doing.
One thing I do is ask what their current thinking is on certain subjects. If this has changed since I left I might remark on it. I hope this helps my mum to see how much doctrine actually has changed in a relatively short time period.
And maybe its working -
I got a shock this week. My young niece (non-JW) had asked my mum for a bible stories book. Instead of a Society book she's gone and bought her a normal Bible Stories book from a shop. She then said to me that the Society's children's books were very inferior in quality to the one she has just bought!
It's only a start but I do feel she is much less judgemental than she used to be.