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

Cassie Laundrie told ABC 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 girlfriend Gabby Petito, who disappeared while the couple was on a cross-country road trip. He was last seen on Sept. 13 or 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 also took questions from a small group of protesters outside her and her husband’s house Monday. In video obtained by Nexstar’s NewsNation, the group asked her if she believes her brother killed Petito and if she knew where he was.

To both questions, she answered, “I don’t know.”

Cassie Laundrie told the crowd her brother flew home around Aug. 17. 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 told ABC she has been cooperating with police. She told the crowd of protesters Monday she hadn’t been able to speak with her family during the investigation.

“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.