Sarah Cotton was back on the witness stand Tuesday in the trial of her ex-partner who is accused of murdering their two young daughters in Oak Bay in 2017.

Cotton began her chilling testimony Monday at the trial of Andrew Berry, who stands accused of second-degree murder in the deaths of four-year-old Aubrey and six-year-old Chloe.

The girls' bodies were found in Berry's Oak Bay apartment on Christmas Day 2017. He has pleaded not guilty to the charges.

On Tuesday, Cotton was once again in the courtroom with Berry as the defence continued its cross-examination of her.

Cotton wept as she told the jury: "That is the biggest regret I'll ever have in my life," referring to her decision to drop her girls off at Berry's apartment on the evening of Dec. 21, to stay with him through noon on Christmas day, even though she suspected the power had been shut off to his suite.

She also became tearful testifying about a message she sent Berry earlier that month with a list of Christmas gifts for Chloe.

She fought back tears as the jury heard about a message she sent the girls' father detailing certain words Chloe would be tested on in an upcoming Christmas spelling quiz.

Much of Tuesday's cross-examination focused on Berry's lawyer, Kevin McCullough, trying to paint a picture of a reasonable co-parenting relationship between Cotton and Berry. McCullough repeatedly asked Cotton about text and email exchanges with Berry regarding co-parenting details.

Cotton rejected the lawyer’s suggestion that she and her ex had face-to-face communications about co-parenting.

She maintained they only communicated about their daughters via emails, texts and phone calls. Andrew would not communicate with me face to face," Cotton told the court.

She told the court she was very upset about a May 2017 court decision that gave Berry 40 per cent custody of the girls.

She was also upset about the custody schedule, which she said involved too much back-and-forth between the two parents.

Cotton agreed with Berry’s lawyer that he was frequently late in dropping off the girls. She said it made her concerned and she found it disrespectful.

During cross-examination Monday, the jury heard about an email sent by Cotton to a friend in May of 2017, expressing her concern about possible gambling debts incurred by Berry.

The email referred to a “gut feeling” Cotton had that Berry had “people after him re: gambling debts.”

Cotton said her concern arose following an incident in which Berry’s living room window had been smashed several months earlier.

Cross-examination of Cotton is expected to continue into the week.