My question - is it reasonable to ask her to provide us with her financials or not even something we should be considering?
First of all, you have handled this situation very well up to this point. Good for you guys!
But do you really want to do this? Ask yourself, "Are we ready to get intimately involved in the financial life of someone else."
If you say yes, you must decide beforehand exactly what you will and will not do and agree upon a course of action between the two of you. Then stick with it.
Some questions to answer:
If she refuses are you prepared to stand firm and offer no additional help?
Are you willing to change the dynamic of your relationship?
What if she has more money than you expected? How will you feel about the money she has been given in the past?
What if her fixed expenses are just barely less than her income? Are you willing to insist on certain changes to be made? What if she is unwilling to change?
What if there is no fixing the budget? Are you willing to take her in? Pay off her debts? Be her caretaker? What if she starts depending on you, are you ready for that?
What if she is 100k in debt? Are you willing to sit back and let her go bankrupt?
This is probably just a start. Make your own list and have a serious discussion between you and your husband.