But the woman, Kathy Lin, has strenuously maintained she is telling the truth, declaring "I know in my heart my husband love me".

Prosecutors allege that late on July 17, 2009, Mr Xie, 50, snuck out of his house in Beck Street, North Epping, travelled the short distance to his brother-in-law's Min Lin's home, and then murdered Mr Lin, his wife, sister-in-law and two young boys as they lay in their beds.

But during the course of his long-running murder trial in the NSW Supreme Court, Ms Lin has repeatedly said under oath that her husband was home in their bed the whole night and could not have killed her brother and his family.

"No, he didn't leave home," Mrs Lin told the court.

But on Thursday, Crown Prosecutor Mark Tedeschi, QC, put it to Mrs Lin that she had lied under oath.

"I want to suggest that when you said Robert was with you, you have lied on your oath," Mr Tedeschi said.

"No, I did give the true answer," Mrs Lin replied.

"I suggest that you have deliberately lied to the court in an attempt to protect Robert," Mr Tedeschi continued.

"No, I didn't," the woman replied.

Later, under cross examination from Mr Xie's barrister, Graham Turnbull, SC, Mrs Lin was asked whether she would lie for her husband if she thought he was the killer.

She said she would not and again declared that her husband had been with her all night.

"No, I know in my heart my husband loves me, he loves ... and he loves my extended family," she said, her voice wavering.

"Every time my mum or my brother ask him to do something I always do that – he never complain."

The trial continues.