The Pennsylvania mom accused of butchering her husband and their 3-year-old son at a Great Wolf Lodge on Monday had drained her husband’s 401(k) to pay for the dream vacation, according to her stepmother — who believes she planned the bloodbath in advance.

Metallica Rose Harrison, 27, tapped husband Charles’ retirement account to pay for the three-week trip to the resort in the Poconos — an early Christmas present for little Tristan — plus a kitchen remodel and other expenses, stepmom April Thetga told The Post.

The family found the sudden financial move “odd,” she said.

“It was definitely odd, but they’d taken their 401(k) out … There was nothing we could do about that,” Thetga said.

She didn’t know whether the couple had borrowed against the retirement account or withdrawn the money outright.

Yet the strange financial choice didn’t raise major red flags for relatives, who had no idea anything was amiss — and even shared a cheerful video call the day before the murders, Therga said.

“She had called and we’re video chatting, and then she says she loved us very much, just like she normally would,” Thetga recalled.

The next day, cops found Charles and Tristan dead from multiple stab wounds after Harrison called 911 and asked for help ending her life.

The mom later raved to cops and paramedics about dreaming of a “happy family” and wanting to join her husband and son in death, according to an affidavit obtained by WNEP.

Before the trip, Harrison had posted cheerful family photos from past vacations.

Thetga described Charles Harrison as a devoted husband and father who rarely, if ever, lost his temper.

“He was a fantastic husband. She definitely got lucky when it came to him. He just was good. He was a good father. I don’t think I’ve ever even seen him fight.”

Relatives still have no idea what could have caused the mom to snap.

“Absolutely it was a loving family. We’re just as shocked as everybody else. We’re not evil people. We want answers. They were always happy,” Theta said.