Sister Sheds Light on Relationship of Runaway Teen Duo

— -- A sister of one of the two runaway Kentucky teenagers linked to an alleged crime spree across the South says "nobody has any clue why" the young couple fled from home.

Dalton Hayes, 18, who lives in Kentucky with his mother, has been on the run for 12 days with 13-year-old Cheyenne Phillips.

Hayes has been seeing Cheyenne "for about three months," but their family didn't know she was 13, according to Hayes’ sister Heather Martin.

"We were under the impression she was 19," Martin told ABC News today. "She would go into the local convenience store and come out with tobacco products. We didn't know she was not an adult."

Hayes and Cheyenne's alleged crime spree began Jan. 4, when they reportedly plowed through a Kentucky cattle farm in an allegedly stolen vehicle. They then allegedly stole a different vehicle, a red 2006 Toyota Tacoma, authorities said.

Cheyenne's father reported her missing, and "that's when things went downhill," Martin said. "My mother texted him [Hayes], 'Where are you at. Cops are looking.' Cheyenne asked 'Why?' My mom proceeded to tell them that, 'You are underage and they want you home.'"

"Maybe they got scared when [Cheyenne] got reported missing," Martin added, but "nobody has any clue why" they left.

Cheyenne and Hayes weren't seen again until Jan. 12, when they allegedly cashed a stolen check at a Walmart in Manning, South Carolina, according to Manning Police.

Hayes’ sister doesn't know why the couple stopped in Manning, South Carolina, and says their family's only connection to the state is vacations at Myrtle Beach.

But they weren’t in South Carolina for long. On Jan. 14, a Henry County, Georgia, resident called 911 after spotting a red Tacoma on the property of an abandoned residence, according to Henry County Police.

The same day, a Silver 2001 Toyota Tundra was reported stolen in "walking distance" from the abandoned Tacoma, the Henry County Police Department reports. But Lt. Joseph Smith of the Henry County Police Department told ABC News today there were no witnesses, so police "can't confirm" whether the teens dropped off the Tacoma and stole the Tundra.

With Hayes and Cheyenne's whereabouts still unknown, his family is using social media "to get it out there that they are missing," according to Martin, and the family is still "calling everybody that we can."

"We don't have any clue at this point where they intend on going," Martin said today.

Lt. Smith of the Henry County Police said today authorities hope "someone will spot the vehicle and call 911."