The Georgia Bureau of Investigation believes Cornwell was abducted by vehicle and is now searching for a white SUV and a tan or gold subcompact car, possibly a Toyota or Nissan, that were seen in the area on the evening of Aug. 11.
For nearly a week, 100 people from more than 15 agencies have been searching for Cornwell.
Richard Cornwell said that his sister's boyfriend was not considered a suspect or a person of interest in the case.
Last year, another woman, Meredith Emerson, disappeared in the same area of northern Georgia. She was found murdered and her killer was caught and put behind bars.
But the people combing the countryside for Cornwell can't believe this is happening again. "We're in a rural community, you wouldn't think it would be like this," one man who lives in the area said.
Friends and family started a "Pray for Kristi Cody Cornwell" group on Facebook. There are more than 3,000 members so far.
One message read, "We love you and Brody and are praying for your safe return."
Cornwell was very active in her community and her church, taking part in a mission to Jamaica with her son Brody.
Anyone with information can call the Union County Sheriff's office at (706) 439-6038, or the Georgia Bureau of Investigation at (800) 597-8477.
CLICK HERE for more information about Kristi Cornwell and the reward for information leading to her whereabouts.
The Associated Press contributed to this report.