They blame the tour company for making the amphibious craft defenseless that scorching hot day, when the captain turned off the overheated engine with 35 passengers aboard and dropped anchor in a busy shipping channel.
The suit charges the city with running a dangerous waterway shared by large commercial vessels and small pleasure craft alike.
And it accuses the tug company that pushed the huge city barge upriver of recklessness for failing to post a lookout despite significant blind spots.
"At the time of the incident, the tugboat was pushing a 290-foot barge upriver both blind and deaf, without caring what or whom they would run over or kill," the lawsuit said.
Dora Schwendtner, 16, and Szabolcs Prem, 20, were killed July 7 when the barge slammed into the stalled duck boat about 150 feet from the Pennsylvania shoreline. The tour boat capsized and sank, tossing all 37 passengers and crew into the dark, fast-moving river.
The 35 others survived, some pulled from the water by firefighters, a passing ferry boat and passers-by on shore.
The lawyer who filed the suit, which seeks more than $50,000 per family in damages, called the deaths senseless and preventable. He questioned why the industry did not make safety changes recommended after a 1999 duck boat accident in Arkansas killed 13 people when a tube disconnected, flooding the hull.
"They sink fast, and they have canopies that become death cages," said lawyer Robert Mongeluzzi, who filed Tuesday's suit. "You need to make the vessels unsinkable. If you can't, you need to take the canopies off. ... They entrap people."
The defendant tour operator, Norcross, Ga.-based Ride the Ducks, challenged the comparison to the sinking of Miss Majestic in Arkansas, which it did not operate, and defended its use of a canopy.
"It is pure speculation that a different canopy or a different reserve buoyancy calculation could have prevented the July 7 ramming of our vessel or altered its tragic outcome," Ride the Ducks said in response to the lawsuit.
Schwendtner and Prem, both described as strong swimmers, attended the same high school in northwestern Hungary. They were touring the United States with a small group of students and teachers through a church-sponsored exchange program and were spending a few days in suburban West Chester.
Each was an only child. The suit was filed by Peter Schwendtner and Aniko Schwendtner, also known as Aniko Takacs, and Sandor and Gizella Maria Prem.
The mate piloting the tug has declined to talk to NTSB investigators, invoking his Fifth Amendment right to avoid self-incrimination. He may testify at some point, but has not yet done so, according to his lawyer, Frank DeSimone.
DeSimone, who has not identified his client, is concerned about the possibility of federal involuntary manslaughter charges. The NTSB's preliminary report on the crash may be finished by the end of August, a spokesman said Tuesday.
K-Sea Transportation Partners of East Brunswick, N.J., which operated the tug, extended condolences to the victims' families but otherwise declined to comment on the suit.
The city does not comment on pending lawsuits, a mayoral spokeswoman said.