I can see why you are in such a pickle. Your wife is in a delicate state right now, and the last thing you want to do is upset her more. I am afraid, however, that silence is not an option. Not unless you want to watch yourselves slip rapidly into financial ruin!
Your wife is engaged in what is called a unilateral (one-sided) activity that affects you, which makes her behavior your business. Unilateral acts are a relationship "no-no."
The only way to handle this with her is to directly confront the issue. Your goal is to be loving and firm at the same time.
Your wife is in a fragile state now, and any hint of criticism will probably upset her (as you sense). So, I would suggest that you begin your confrontation with a support statement. Something like, "Honey, I know how upset you are about your mother, and your loving caring nature is one of the things that I love best about you..." If these words don't feel right, find your own.
Then, continue with the confrontation (my X, Y Formula) in which you describe what your wife is doing and how you feel about it. For example, "I notice that $800 has been charged to the credit cards in the last four weeks, and I am upset because if we don't get our debts under control we will end up bankrupt...Then how can we help your mother?"
Next, add what I call the "Suggestion for the Future" in which you give your wife guidance regarding what you envision as a solution to the problem. You might say, "I want us to sit down together, look at our income and debts, and agree on an amount of money that we can afford to give to your mom each month, so that we can take care of her without ruining ourselves."
Once you open the discussion regarding your wife's mom, you will find it helpful to get your wife talking about her feelings of fear, helplessness, and anxiety over the prospect of losing her mother. Until she owns these feelings and discharges them in words, she will continue to act out in ways that discharge her feelings (running up credit card debt). If she can talk about these feelings, you have a better chance of getting her spending under control.
After the dust has settled regarding this monetary issue, you are going to need to address the larger issue here: that she makes unilateral decisions that affect you and the family. She needs to learn to consult with you in the future before making any decisions or taking any actions (including spending money) that affect you and the family without talking with you first.
Good luck with this touchy situation. If you stay cool, and speak in a loving way, you should be fine.