Shanann Watts' husband Chris Watts killed her, their two young daughters and their unborn son in August

When Shanann Watts blew out the candles on her birthday cake on January 10, 2018, with her two young daughters by her side, she had no idea it would be her last birthday — or that all three of them would be dead in the coming year.

In a video posted online of her 34th birthday celebration, Shanann’s little girls, Bella, 4, and Celeste, 3, sang a sweet rendition of Happy Birthday to their mom as Shanann’s own mother, Sandra Rzucek, joins in via Facetime on Shanann’s phone.

Shanann’s husband, Chris Watts, 33, can be heard singing in the background.

When the song ends, Bella blows the candles out on the round cake adorned with a butterfly on the side.

Chris and Shanann Watts with their daughters, Bella, left and Celeste, right
| Credit: Facebook

“What!?” Shanann says in mock surprise to Bella. “Did you blow my candles out?”

Bella giggles as Celeste shouts something about candles in toddler talk.

Had Shanann lived, she would have been 35 today.

“She loved everybody,” her mother told ABC News in December in an interview with 20/20. “Her love for children and friends was amazing.”

Bella, Celeste and Shanann Watts
| Credit: Shanann Watts /Facebook

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Last summer, Shanann and her daughters spent six weeks in North Carolina with her parents and brother before returning to the Frederick, Colorado, home she shared with Chris on Aug. 7. She left home again to attend a quick work trip on Aug. 10, returning home just before 2 a.m. on Aug. 13. Hours later, Chris killed her and the girls.

Authorities still do not know exactly what led Chris — who was having an affair with a co-worker — to kill his whole family that fateful morning, after which he went on TV news to plead for her safe return.

“If he was this happy and wanted a new start, get a divorce,” Weld County Prosecutor Michael Rourke said.

“That’s the big word — why?” Shanann’s father, Frank Rzucek told 20/20. “He must have snapped because there’s nothing else I could figure out what happened to him.”

Sometime after Shanann returned home, Chris strangled Shanann, who was 15 weeks pregnant with her unborn son, Niko, and smothered his two little daughters.

Celeste and Bella Watts
| Credit: Facebook

Chris loaded their bodies, one at a time, into the back of his truck and drove out to a remote oil site where he worked.

He buried Shanann in a shallow grave and shoved his daughters in nearby oil tanks through a hatch that was only eight inches in diameter, authorities said.

Bella had scratches on her body from being shoved through the hatch. Investigators found a tuft of blonde hair on one of the hatch openings.

Candlelight vigil for Watts family
A candlelight vigil honoring Shanann and her girls in Frederick, Colorado, after their murders
| Credit: RJ Sangosti/The Denver Post/Getty

On Nov. 6, Chris pleaded guilty to murder, the unlawful termination of a pregnancy and tampering with a dead body in exchange for an agreement that prosecutors would not pursue the death penalty, the Weld County district attorney’s office said.

Thirteen days later, he was sentenced to five life sentences, three to be served consecutively, without the possibility of parole.

In December, he was tranferred to an out-of-state prison “for his own safety,” a source close to Watts tells PEOPLE.