TAMPA, Fla. (WFLA) — Brian Laundrie’s sister has urged her brother to surrender and her parents to “come clean” if they have anything to do with his disappearance.

Cassie Laundrie told ABC’s Good Morning America that she doesn’t know where her brother is, but if she did she would “turn him in.”

Brian Laundrie remains the sole person of interest in the death of his fiancée Gabby Petito who disappeared while the couple was on a cross-country trip. He was last seen on Sept. 14, days before Petito’s remains were found in Wyoming. Laundrie’s parents told police he went for a hike in the Carlton Reserve and never returned.

Cassie Laundrie said her brother and parents visited her on the day he returned home from the trip.

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

She said she last saw Brian on Sept. 6, while their family was camping at Fort De Soto Park.

“We just went for a couple of hours and we ate dinner and had s’mores around the campfire and left,” Cassie recalled. “There was nothing peculiar about it, there was no feeling of grand goodbye.”

“I’m frustrated that, in hindsight, I didn’t pick up on anything,” she added. “It was jut a regular visit.”

Cassie said her brother’s disappearance is considered out of character.

“I worry about him. I hope he’s ok. And then I’m angry and I don’t know what to think,” she said. “I would tell my brother to just come forward and get us out of this horrible mess.”

Cassie told ABC she has been cooperating with police.

“I don’t know if my parents are involved,” she said. “I think if they are, then they should come clean.”

When asked about the dispute involving Petito and Laundrie in Utah, in which a 911 caller reported witnessing Laundrie “slapping” Petito, Cassie told ABC it was common for the couple to argue and take space from each other, but she never witnessed any signs of domestic violence.

Cassie said she was also mourning Petito’s loss and shared condolences with the Petito family.

“They deserve answers,” she said.

On Monday, a group of protesters asked Laundrie whether she believed her brother killed Petito.

“I don’t know,” she responded.