The sister of a Kentucky woman last seen more than a week ago believes her live-in boyfriend knows something about her disappearance, despite him denying involvement and police saying he has been extremely cooperative with investigators.

Crystal Rogers was last seen by Brooks Houck, her live-in boyfriend, on July 3, according to the Nelson County Sheriff's Office in Kentucky. Her sister, Brooke Ballard, said she has questioned Houck's claims that Rogers, 35, stayed up late that night and was gone when he awoke the next morning.

"The stories just don't add up. You just don't go to bed one night and not know she's gone and then not worry when she left the baby," said Ballard. "He has not offered once to search -- or help -- or do anything for the family."

Houck said he had nothing to do with the disappearance of Rogers, a mother of five including his 2-year-old son. He took a polygraph test and told HLN’s Nancy Grace the results were inconclusive.

"I'm 100 percent completely innocent in this,” he said, adding that his search efforts have been done "behind the scenes."

”I want the emphasis to remain on Crystal’s safe return," Houck said.

Investigators haven't named Houck or anyone else as either a suspect or person of interest.

Since Rogers was last seen, police and volunteers have worked to retrace her steps, scouring farmland and woods in the area.

But the only sign of Rogers so far was her car -- a maroon Chevrolet with a flat tire -- found along the side of Kentucky's Bluegrass Parkway, an area loved ones told police she rarely traveled. Her keys, purse and uncharged cell phone were inside the vehicle.

A $71,000 reward is being offered for her return.