The allegations emerged as Timothy Jay Vafeades, 54, made an initial appearance in US District Court. The charges include kidnapping, transportation for illegal sexual activity and possession of child pornography, and they could bring a life sentence if he is convicted.
An arrest warrant filed in Salt Lake City, Utah, says Mr Vafeades kidnapped a 19-year-old female relative who had come from Florida in May last year to work with him on his truck.
After a week, the teenager told him she wanted to go home but she later told authorities he strangled her until she blacked out and used threats and violence to keep her with him for the next six months while they travelled across the country.
After his arrest, a second woman went to authorities to report she had also been held captive in Mr Vafeades's truck.