Yes, I would.
I turned in my father's brother and his son, (my first cousin), to the police and the IRS when I discovered they were embezzling and hiding money stolen from my family business.
Turned out my father knew they were doing it too, but pretended not to. I wanted to turn my father in as well, but that would have done irreparable harm to the financial well being of the company. Too many families' income and futures were at stake not to act. Unfortunately the actual paper trail leading to my father is gone. So for now we tolerate one another.
(Father paid all fines for my uncle and cousin. Charges were dropped.