I thought Nick ratted because Jr implicated him in the murder of John Fecarotta. They had a bloody glove at the scene and Nick had a bullet in his arm from the struggle inside the car over the gun before he chased John down and shot him in the head. They had his blood on the glove to match with DNA and after Frank tipped off the feds they dug the bullet out of Nick's arm and it was a match for the gun used in the murder. Feds were charging Nick with a Death Penalty case to get him to flip too. Frank really fucked him over and kind of forced him into flipping. Plus wasn't Marcello only paying him something like $4K a month to keep his mouth shut while he was in prison? The guy was worth and earning millions.

Frank Jr has always struck me as someone who knew where a lot of his father's stash spots were and figured if he flipped all that would become his. The Feds would have gladly made that trade to let him keep the millions Sr. had squirreled saw in exchange for ultimately getting the first made guy to flip and put the Boss in jail for life. Feds could always claim deniability on knowing there was $$$$ out there, what do they care?