On the Friday night broadcast, Sandra and Frank Rzucek also revealed how they felt about Chris before the gruesome nightmare began.
As RadarOnline.com has extensively reported, cold-blooded Chris murdered pregnant Shanann, 34, and their two young daughters, Bella and Celeste, in August at their Colorado home.
Chris had strangled his wife and smothered the girls; he buried Shanann in shallow grave in a crude oil field where he worked and put their daughters' bodies in oil tanks. Bella and Celeste's skin had fallen off after their little corpses were discovered.
The murders devastated Shanann's parents and her mom Sandra told 20/20, "You know someone asked me a question one day, 'Sandy how are you standing?' Well, I have a fight to fight. That's my daughter. I gave her life, and I'm still going to fight for her… you know"
"Those grandchildren… they were my everything, they were my only grandchildren in the whole world," Sandra said, getting teary-eyed while sitting next to her husband Frank for the emotional TV interview.
Sandra called Chris the "devil," confiding, "We did scream at God, yes we did.
"The devil comes to you in any shape, size and form and can be transformed into anything."
Frank mused about how Chris became the man who killed his family, "That's the big word - why? He must have snapped because there's nothing else I could figure out what happened to him."
Sandra and Frank remembered how Shanann had bravely overcome the risks of her lupus diagnosis to have children.
"She was fun, full of life. She did so many things. She had her hand in everything," Sandra proudly recalled.
Sandra said her daughter, suffering from the autoimmune disorder and expecting her third child, a boy, with Chris, "never gave up" and kept a "vision board."
According to Shanann's dad Frank that was "to make sure her children had everything they ever needed."
The Rzuceks said that Chris had appeared to be extremely distant before the murders.
According to Sandra, Shanann "just couldn't understand why he was so cold."
It was a far cry from most of their marriage when Chris "did everything he could for her to make her happy. It was amazing love," Sandra remembered.
Shanann's mom added, "He never raised his hand, voice….it was an amazing relationship so I just don't know."
She said her daughter was "clueless" that Chris had been cheating on her as were her family members.
We pay for juicy info! Do you have a story for RadarOnline.com? Email us at email@example.com, or call us at (866) ON-RADAR (667-2327) any time, day or night.