KINGSTON - A Montreal family accused of killing three children concocted a story that could help absolve them of guilt and told a surviving child to go along with it -- but he slipped up, prosecutors alleged in court Tuesday.

Mohammad Shafia, 58, his wife Tooba Yahya, 42, and their son Hamed, 20, have told police that the night of the deaths, their eldest daughter came into their motel room to borrow the car keys.

The Crown alleges the now-deceased children were killed before the family checked into the motel, and that they made up the story about the car keys to place the girls at the motel alive, which would fit with their assertion that the deaths were a joy ride gone wrong.

Shafia sisters, Zainab, 19, Sahar, 17, and Geeti, 13, and Rona Amir Mohammad, 52, Shafia's other wife in a polygamous marriage, were found dead in a car submerged in a canal in Kingston, Ont., on June 30, 2009. The family was on their way home from a trip to Niagara Falls, Ont.

It's alleged they were killed over family honour. Shafia, Yahya and Hamed have each pleaded not guilty to four counts of first-degree murder.

Another brother to the girls, who cannot be identified by court order, was under cross-examination as a defence witness Tuesday, and Crown attorney Gerard Laarhuis hammered him about his statement to police that Zainab came to the motel room that night to borrow his cellphone.

Laarhuis suggested that never happened, and further, that inconsistent statements to police that it might have been his mother who wanted to borrow his phone, point to a mistake on his part.

"I'm putting to you, that's where you're getting confused, because the story was supposed to be Zainab came and asked for keys, but you got confused," Laarhuis said. "You got mixed up and you said she came and asked for a cellphone."

The brother denied the suggestion.

Laarhuis pointed to an intercepted phone conversation the brother had with Hamed and their mother the night he was removed from the family home by authorities, which was also the night before the accused were arrested.

He is heard saying they are "100 per cent caught," a statement he says now he was just repeating from the police, and telling them what he told police in an interview that day. Laarhuis suggested they were trying to get their story straight.

"The reason that you're confirming this with Hamed and your mom is because you know there's a problem with that part of the story," the lawyer said.

"No, not really," the brother said. "I'm just trying to help them" tell the truth," he said.

"That's the remarkable thing about the truth," Laarhuis replied. "You don't need to remind people what the truth is."

The lengthy cross-examination was often testy or defensive, and riddled with several objections from the three defence lawyers. The brother was unclear about many answers, which sometimes elicited chuckles from members of the public watching the trial.

At one point, the brother said he went to visit the site of the deaths once afterward, to see where his sisters had died. Laarhuis suggested a second time he visited, and the brother responded: "Uh, yes. I'm not sure. I don't recall. I don't remember it."

"You said yes ... What's that about?" Laarhuis asked.

"Maybe I was back there in Kingston, but not on the (specific) site," the brother said.

Laarhuis also suggested the family had stopped at the site of the deaths to use the washroom at the beginning of that trip, which Shafia admitted on the stand last week, but the brother said he wasn't sure. He said he remembers stopping in a park, but wasn't clear if there was water at that park, saying, "Uh, yes. I don't know. I'm not sure."