Our CPA told a family member who also uses them to do their taxes, how much money we make and what our net worth is. Was this illegal? Thank you!

No I do not think so; A tax return is a document you must file with the government showing how much income you earned in a particular year.
you?ll need to start preparing your tax return is the total amount of income you earned during the year. While the tax laws may change from year to year, the basic information you need to do your return or take to your tax preparer doesn't change that much from year-to-year;a good accountant will save you money if/where they can