She is earning considerably more than the president of the República .... :
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It is certainly possible to spend here 4000 USD for a family per month, at least in Capital. But that would include many expenses that you don't seem to have (private medicare, private school, cars etc).
Just the fact that you are asking the question means that you basically already know the answer. I am not a counsellor or something, but how do you manage to live together with them (not only with your GF, but also mother in law and family) knowing that they take advantage of you?