How can I stop my FMIL from taking some of the money from our cash gifts?

Just don't give people those cards that demand money. And if she already gave them to people, just don' t give her any of it. How is she getting her hands on your money anyway?? If you think her son has no backbone now, think about how you'll feel after dealing with all of this for 30 years or more. Do some thinking before you marry into that family. And by no means should you let her around any of your stuff, including wedding gifts.