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Authorities have been searching for the 23-year-old since his parents reported on Friday, Sept. 17, that they had not seen their son since Tuesday, Sept. 14.
Law enforcement has named Laundrie a "person of interest" in the homicide of his 22-year-old fiancée, fellow Long Island native Gabby Petito, who was from Blue Point in Suffolk County.
"I would tell my brother to just come forward and get us out of this horrible mess," Cassie Laundrie told ABC in the interview that aired Tuesday, Oct. 5 on "Good Morning America."
Cassie Laundrie also said that she would turn her brother in if she knew where he was.
Petito's remains were found in a Wyoming national park on Friday, Sept. 19. She had been traveling across the country with Laundrie in her van prior to her disappearance.
Brian Laundrie returned to his home in Florida on Wednesday, Sept. 1, without Petito and did not cooperate with investigators after his fiancée was reported missing.
Cassie also said in the interview that she has been cooperating with authorities, and called on her parents to do so as well.
"I don't know if my parents are involved," Cassie told ABC News. "I think if they are, then they should come clean."
Cassie said she last saw her brother on Monday, Sept. 6 when the family went camping in a park in Pinellas County, Florida, five days after Brian returned home alone to North Port, Florida, in Sarasota County, after he and Petito went on a cross-country van trip.
"We just went for a couple of hours and we ate dinner and had s'mores around the campfire and left, and there was nothing peculiar about it," Cassie told ABC. "There was no feeling of grand goodbye. There was no nothing."
"I'm frustrated that, in hindsight, I didn't pick up on anything. It was just a regular visit."
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