The horrific video, which surfaced on Monday, showed Wuttisan Wongtalay hanging his daughter before taking his own life.
The fact that it was up on Facebook for over 24 hours has sparked a debate about how to keep this graphic content off the internet, but until now, no one realised that the girl's mother had watched the entire thing.
"I was with my older brother and he was logging onto his Facebook," Jiranuch Trirat, told AFP on Thursday from a Phuket temple.
"He was scrolling down and suddenly we saw the live broadcast."
"I turned to take a look and saw him (Wuttisan) drop my daughter with the rope and I couldn't continue to watch."
They quickly realised the what was happening and alerted the police. The bodies were found hours later.
As for the Facebook debate, Trirat said, "I don't blame Facebook. They are not part of the problem, we can choose to broadcast happiness or sadness."
What prompted Wongtalay to commit the murder and suicide remains a mystery, but Trirat did reveal, "He often abused my son Sanook."
Phuket News reports that the events took place after 20-year-old Wongtalay threatened to kill Trirat. She ran away, leaving their baby Natalie with him.
Ms Jiranuch told police that he had found her phone, and accused her of cheating on him.
“At 3am yesterday he checked my phone and threatened to kill me,” she said.
“I was very afraid and ran away from the house and left Natalie with him. I returned home late in the afternoon and they were not there. I called him to bring Natalie back to the house, but I could not get in touch with him."
“The video clip, which is about four minutes long, was posted at 5:45pm yesterday,” said Lt Col Sanit Nookhong from the Thalang Police on Tuesday.
“You can hear the girl crying and finally the crying stops,” he said.
A statement from Facebook about the incident described the events as "appalling".
"Our hearts go out to the family of the victim," it said.
"There is absolutely no place for content of this kind on Facebook and it has now been removed."