(NBC NEWS) – Tonight, insiders speak exclusively to “Dateline” about Idaho couple, Lori and Chad Daybell, who are facing multiple charges after the remains of Lori’s children, JJ and Tylee, were discovered in Chad’s backyard.

The two-hour special reveals new details on the mysterious deaths of Chad’s former wife, Tammy, and Lori’s former husband, Joe. In an exclusive interview, Tammy’s sister Samantha opens up about learning from Chad that Tammy had died in her sleep.

Here is a preview of Keith Morrison’s report:

This is Tammy’s only sister, Samantha, and Samantha’s husband, Jason.

SAMANTHA GWILLIAM: I got a phone call from Chad tellin’ me that she had passed away in her sleep.

KEITH MORRISON: How do you process something like that?

SAMANTHA GWILLIAM: (CRYING) You don’t. I mean, like, there’s just so many thoughts running through your head. And a lot of it was, like, “No. That–that didn’t happen.”

KEITH MORRISON: What did Chad say happened?

SAMANTHA GWILLIAM: That–that she’d been sick and had a coughing fit and passed away.

Tammy Daybell was just 49 years old, slim, fit and as far as Sam knew, perfectly healthy.

KEITH MORRISON: Were you surprised to discover there had been no autopsy?

JASON GWILLIAM: You know, the question had been asked by my wife to Chad on Saturday when she passed and his response was, “Well, she’s already over at the mortician. They said there was no need to have an autopsy.”

Watch Friday’s “Dateline: Mommy Doomsday” tonight at 9 p.m. on NBC4.