A Montreal woman accused of killing her three daughters and her husband's other wife broke down in court Monday while testifying about her polygamous marriage.

Taking the stand in her own defence, Tooba Yahya was in tears as she discussed her relationship with Rona Mohammad, the first woman her husband, Mohammad Shafia, married in Afghanistan.

Mohammad couldn't have children so Shafia took a second wife. Yahya said that while she was pregnant with her third baby, she promised Mohammad the child would be raised as her own.

Mohammad, 52, and Shafia sisters Zainab, 19, Sahar, 17, and Geeti, 13 were found dead in a car at the bottom of a Kingston canal on June 30, 2009.

Yahya, 42, Shafia, 58, and their eldest son, Hamed, 21, are accused of killing the four women over family honour.

All three have pleaded not guilty to four counts of first-degree murder.

The trial heard from defence witnesses, including the father, before court took a break for the holidays.

The jury is now hearing some of the background information on how Yahya came to marry Shafia when she was only 17 and how their family left Afghanistan.

While Yahya described a well-functioning household, court has heard excerpts from Mohammad's diary, in which she wrote that Yahya had threatened her and beat her along with Shafia.

Yahya denied that, saying her husband's first wife was very happy and they all got along well.

"Any person can write whatever they want," Yahya said.

The defence has said it expects to have another two weeks of testimony but it has not named all their witnesses.

The Montreal family had been heading home after a trip to Niagara Falls, Ont., when the family says the accident occurred. The Crown has alleged that the deaths were staged to look like their car accidentally plunged into the canal.

Crown prosecutors have alleged in court that the elder Shafia in particular was upset that the girls were dating boys and telling authorities they didn't feel safe at home. They say the girls were killed to restore family honour.

During his testimony, Shafia insisted that it was an "impossible" claim to suggest he would kill his own daughters.

He said that murder is not permitted by his religion. He also said that while honour was important to him, it was not important enough to drive him to kill and that there was no honour in our society for someone who kills his own children or wife.

On Monday, Yahya contradicted earlier testimony that suggested her older daughters were pressured to wear a hijab, saying the girls were not subject to rules governing what they could wear. She said each of her children was subject to the same curfew.

Yahya said she often went so far as to hide her children's transgressions from their father to protect them.

"I knew (his) habit that if there was a small thing he used to make it a big thing," Yahya said. "He used to go on and continuously he was just swearing at them and continuously talk about that for weeks."

Yahya also denied that a suicide attempt by Sahar was the result of pressure to wear a hijab and subsequently being shunned by her parents.

Rather, Yahya said her daughter regularly threatened to kill herself if she couldn't do what she wanted, and the attempt in question occurred after Sahar became angry when her sister wore her pants to a party.

Yahya dismissed a summary of the incident found in Mohammad's diary, in which she said Yahya reacted to Sahar's suicide attempt by saying her daughter could "go to hell. Let her kill herself."

"No, I never say anything like this," Yahya said. "I was afraid and I was crying."