The warmest of welcomes to you at such an incredibly stressful, frustrating and tormenting time in your life.
As a guiding light, Tony offers brilliant advice. Seeking the assistance of another guide (a financial guide) could be an option. As he mentions, it could well be worth going along with or without you wife. You sound like the only one in the family who's seriously concerned. If you can clearly see where this is heading, if the overspending continues, your concern is completely understandable.
Perhaps a part of the challenge you face could involve developing a different vision. For example, could you visualise and feel some sense of relief if your mortgage repayments were deducted directly from your pay? Maybe this would also act as a wake up call for your wife, as there's no longer $9000 coming into the bank account each month, a massive chunk of income or what the family currently sees as 'spending money'. I think sometimes it makes a huge difference when we speak to the right visionary in regard to the challenge or challenges we face, someone who can lead us to see the best way forward.
A concerning or stressful lack of money is a horrible thing to feel, that's for sure. On the other hand, when you can feel plenty of cash there, it feels fantastic. I admit to being a gal who's imagined some financial fantasy in the past, imagining a bank balance that never goes down. Eventual financial stress, due to a very real low bank balance, can be a massive wake up call. While your family lives a fantasy lifestyle or style of life, you face an inevitable reality, great hardship down the track. I imagine there is something that will wake your wife up. The question is 'What will it be?'. Maybe a financial advisor can see a way. Perhaps putting part of the $20,000 into a fixed term account, while having the mortgage directly debited from you pay, may lead her to see that the money she imagines will always be there is a part of her imagination.
