Just before her death on Thursday, Dec. 18, Cristina Grossu Biffle texted her mother Cathy Grossu, writing, “We’re in trouble,” Cathy tells PEOPLE.

“She texted me from the plane and she said, ‘We’re in trouble.’ And that was it,” Cathy says, crying. “So we’re devastated. We’re brokenhearted.”

The crash killed seven people in total: Greg, 55, Cristina, their son Ryder, 5, and Greg’s daughter Emma, 14 (who he shared with ex-wife Nicole Lunders); plus Dennis Dutton and his son Jack; and Craig Wadsworth.Greg and Cristina Biffle and their son.Greg Biffle/Instagram
The group was heading out for a “birthday trip” in Florida, Cathy says.

“To think that they would be killed on a birthday trip, that was just such a fun time for the family,” she says. “And to see the horrific way that it ended, it’s just, it is so hard to bear. I cannot believe they’re gone.”

Just the day before, Cristina and Greg were at her house.

“I don’t remember what the last words that I said to my daughter or to Greg or to my precious Ryder,” Cathy says. “I don’t remember. I know we hugged, but I don’t remember those last words and that’s going to haunt me. But they were happy.”

Cathy says her grandson Ryder and step-granddaughter Emma, as well as Greg, “had so much life” and were “doing so many good things” before the tragedy.

“They embraced every aspect of their life and every moment,” she shares. “And it’s such a loss. They touched so many people’s lives. It is so hard to bear. I cannot believe they’re gone.”

Cathy says that one of her last conversations with her daughter was about Santa letters that Cristina would send “at her own expense” to families in need.

“She said, ‘Mom, can you go pick up the last 17 letters that are at Staples? I want to get them in the mail before I get on the plane tomorrow,’ ” Cathy shares.

“So I ran over and got ’em and took them to their house, and when they got home last night, she put them in envelopes and finished them all up to get them off to those last 17 families to bring joy to them, and that would be the last thing that she would’ve done.”