She said that her mother learned of the affair in 2013 but appeared to have come to terms with it. But, contradicting the maid’s testimony, she said that her parents shared their two cars.

Ms. Khaw, who said her deceased sister was her “soul mate,” described her sobbing father breaking the news of her mother and sister’s deaths in a phone call.

“It was a really strange sound, and one I’ve never heard before,” she told the courtroom. “It took me a while to realize that my dad was crying on the phone.”

The remaining family members stayed together in their home after the deaths, hiding from photographers and reporters, Ms. Khaw said, and the children tried not to leave the father home alone. At night, they pulled mattresses into one room.

“He just looked so broken,” she said.

At the end of her long testimony, she asked the judge if she could remain in the courtroom to “support her father,” who had been weeping as she described the aftermath of the deaths and the pressures the remaining family members faced. Her younger siblings also appeared in the courtroom last week, smiling and waving at their father before court sessions began.

Dr. Khaw had long exhorted his children to excel despite any hurdles they faced, including learning disabilities or mental health issues. Ms. Khaw said she was told she had depression, anxiety and attention deficit disorder at 16. Lily got a diagnose of dyslexia and attention-deficient hyperactive disorder when she was 6, and her eldest sister described her behavior as “impulsive.”

In the testimony, Ms. Khaw said that her father had exerted immense pressure, trivializing their struggles to focus on academics and rebuking them for not trying hard enough. “My father discouraged me from taking anti-depressants because he didn’t believe that I was depressed,” she said.