The short answer is – don’t try and explain to them anymore.
Many people around CGs find denial easer to live with because to accept means facing hard truths.
Unasked for, unconstructive criticism tells you a great deal more about the person doing the criticising. I don’t like the expression ‘there are two sides to everything’ which always seems to be said by someone wanting to score points in the blame game. If there has to be an assessment of the sides then – your wife is a compulsive gambler and you are not. You know that you have been a good husband and therefore there is no need to justify yourself to your wife’s sister.
I know it is hard James to avoid things like your wife’s sister making her observations but she is not central to your life – that position is occupied by you and your daughter. You will probably have more unhelpful things said before everything is sorted out but provided you remain focussed on that which matters you will do ok.
Just a thought James – you posted this as I was running a group, it would have been good to communicate in real time.