Brian Laundrie’s sister spoke out for the first time about his disappearance and the Gabby Petito case in an interview with "Good Morning America" that aired Tuesday. 

In the interview, Cassie Laundrie insists that she does not know where he is, stating that if she did, she would fully cooperate with law enforcement. 

 "No, I do not know where Brian is. ... I'd turn him in," she says. 

Cassie Laundrie also said in the interview that she was worried about him, but hinted that she was in the dark as anyone else when it came to not only where he was, but what role he may have potentially had in Petitio's disappearance and death. 

 "I hope my brother is alive because I want answers just as much as everybody else," she said. 

The interview also served to clarify the last time Cassie Laundrie said she had seen or heard from her sibling, indicating that the last time she had contact was during a family camping trip on Sept. 6--and that in hindsight, she wished there had been a more obvious sign that she could have picked up on about what had happened. 

"We just went for a couple of hours and we ate dinner and had s'mores around the campfire and left. There was nothing peculiar about it. There was no feeling of grand goodbye. There was no nothing, "I'm frustrated that, in hindsight, I didn't pick up on anything," she added. "It was just a regular visit."

She also indicated that maybe if her brother had come to her first after going home, that things could have ended better for the entire family. 

 "I really wish he had come to me first that day with the van," she said, "because I don't think we'd be here."

Overall, Cassie Laundrie said that if her parents were involved in any way with her brother's disappearance that she thinks they should come forward, but most importantly, she urges her brother to do the same so they can all move on with their lives. 

 "I would tell my brother to just come forward and get us out of this horrible mess," Cassie said.