Ratting out a blood relative to the authorities is the lowest thing you can do. Except for sleep with the spouse/girlfriend/boyfriend of a blood relative.
"Uh, hi operator, we had a policeman come to school today with a poster about illegal guns, you know, machine guns and assault rifles and stuff.... and well... my daddy has one of those long black rifles with the handle thing on top. He has it hidden up in the attic of the garage, I saw him put it there last year. And I know how dangerous they are, and I don't want to see him get in trouble, so....
"Don't worry son, we have police units on the way to... help you. You did the right thing."