Do you think that maybe she has a psychological problem with compulsive spending? It seems that she has a very hard time budgeting and keeping enough money around for emergencies. I know that sometimes, when I was feeling down and the account had money in it, I had a tendency to buy things to raise up my spirits even though I really couldn't afford them. It took a lot of discipline to truly refocus my happiness on spiritual peace rather than material things.
I don't know if her father and step-father are aware of her spending sprees. Even if you were withholding money from her, it seems to me that she's currently living a life she simply cannot afford. From the looks of it, there is financial irresponsibility there. Just because you have access to money doesn't mean that you have to spend it all right away.
Another possibility is that she may be gambling with the money and losing. Is gambling available where she is? I just find it odd that so much money can disappear so quickly without seeing any new significant purchases.
Is she currently in counseling? Is this something the both of you could bring up? Sounds to me like a mountain of debt is accumulating and, one day, it will come back to haunt both of you. You obviously sound as if you want to put your marriage ahead of financial wealth. On the other hand, she seems to think that the fruits of financial wealth is more important than the marriage.
Don't throw in the towel just yet but see if you can get her family to see the expenses she has had over the course of the sitch. They may be able to convince her to get help for this compulsive behavior. Obviously, she hasn't resolved all of her issues since she is looking for happiness in a lavish lifestyle rather than in day-to-day life.
