Eventually, the sons of "Big John" caved in to the FBI in interrogation. While the tip of the former girlfriend made the whole family prime suspects in the first place, they did not have anything on them until the sons decided to talk (and get immunity for their involvement in exchange).
However, the two sons only decided to talk after the police made each one believe that the other was ratting him out. Classic interrogation technique that I've seen in countless procedural tv shows and movies.
Eventually, the sons of "Big John" caved in to the FBI in interrogation. While the tip of the former girlfriend made the whole family prime suspects in the first place, they did not have anything on them until the sons decided to talk (and get immunity for their involvement in exchange).