I also applaud the way you and your husband handled this.
However, even though it is none of my/our business, I can think of only a few things that would involve your mother signing papers, and all involve money in some sense --whether outright or a release or........ If that was the case, as least your mother would have a right to know what SIL does in general. I also don't find it a rude question to ask someone, and it can usually be answered with a very simple "I'm in the retain/banking/construction/whatever field" or simply "I'm not working at the moment to stay home with and enjoy the children". It does not have to ever be asked for specific details nor answered with specific details.
That being said, I'm glad you were able to enjoy your Easter dinner.