Cassie Laundrie says she last saw her brother at a Florida campsite with her family days before he was reported missing.
She said she had no idea anything was amiss and didn't realize at the time that he had returned home from his road trip without Petito.
"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. "I worry about him. I hope he's OK, and then I'm angry and I don't know what to think."
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She pleaded with him to come forward.
"I would tell my brother to just come forward and get us out of this horrible mess," she said.
Separately, Laundrie family attorney Steven Bertolino confirmed to ABC News that the family now believes Brian Laundrie left to hike the Carlton Reserve on Monday, September 13.
"The Laundries were basing the date Brian left on their recollection of certain events. Upon further communication with the FBI and confirmation of the Mustang being at the Laundrie residence on Wednesday, September 15, we now believe the day Brian left to hike in the preserve was Monday September 13," Bertolino said.
Previously they had told investigators he left on Tuesday, September 14.
He also confirmed earlier on Tuesday that Brian Laundrie flew home to Tampa from Salt Lake City on August 17 and flew back to Salt Lake City on August 23 to "rejoin Gabby."
Bertolino said Brian flew home to "obtain some items and empty and close the (couple's) storage unit to save money as they contemplated extending the road trip."
Bertolino said the couple paid for the flights together, as they were sharing expenses.
To place this in the timeline, the Moab, Utah, domestic incident captured on bodycam happened on August 12, and the possible last sighting of Gabby alive at Merry Piglets, the Tex-Mex restaurant in Jackson Hole, Wyoming, where Brian was seen getting angry with the staff and Gabby was crying, was on August 27.
The developments come after a hiker claimed he saw Laundrie in a parking lot near the Appalachian Trail Saturday.
Dennis Davis says he saw Laundrie in a parking lot near the trail, and the two even had a conversation.
Davis said he has no doubt in his mind who he was talking to, but at the time, he didn't know what Laundrie looked like.
He says the man he mentioned having a fight with his girlfriend.
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"I started talking to him, and immediately I could tell that there was something wrong with the guy," Davis said. "I thought that maybe he was on drugs or something at first. What he said to me at the start was, 'Hey, man, I'm lost.'"
Petito, 22, was found dead in Wyoming on September 19.
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