How risky was this illegal behavior by my ex in laws?

May they rest in peace; I miss them so. Anyway , after my father in law retired his wife was doing all the business end of the household such as their income taxes. One day when my ex and I were over there she made a remark about “since we live on a fixed income.” My ex said, “fixed income? What about all that interest you’re making on your stocks?” and she said, “oh, the government isn’t going to arrest an old lady like me.” They never did arrest her and she saved a lot of money.
Was this risky behavior on her part or could it be fairly common?