(Photo: Twitter/Jessica Simpson)Jessica Simpson shared a photo taken during the wedding between Cacee Cobb and Donald Faison on Saturday, Dec. 15, 2012.

Jessica Simpson is reportedly concerned about claims that her fiance Eric Johnson's ex-wife is writing an explosive tell-all book.

The singer, who is currently expecting her second child with Johnson, reportedly met Johnson in 2009 while the former NFL star was still married to his now ex-wife Keri. A report claims that the pair allegedly began dating despite Johnson still being married at the time, which Keri reportedly intends to expose.

"Jessica can't believe someone would resort to something so low," a source reportedly told In Touch magazine. "She doesn't want anyone to know all this about her."

Johnson, 33, first met Simpson at a mutual friend's house in Los Angeles, where he had allegedly brought Keri along as his date. He and Keri, a freelance stylist, separated in October 2009 after five years of marriage and it is understood that Johnson began dating Simpson approximately six months later.

"If Jessica hadn't come along, Eric and Keri would still be married," the source continued. "Keri has always wanted to write a book about her divorce; [subsequently] she hasn't talked or sold any photos … yet."

This is not the first time that Simpson, who was previously married to singer Nick Lachey, has been accused of engaging in an inappropriate relationship with Johnson during his marriage to Keri. In October, an insider openly condemned him and had a stern warning for Simpson.

"He's a two-timing cheat! He was still having sex with Keri after he started seeing Jessica. She needs to think twice before marrying him. He's already cheated on her once- he'll most likely do it again," an unidentified friend reportedly told Star magazine.

The couple, who has been together for two years, announced their engagement in November 2010, but they was forced to postpone their anticipated November 11, 2011 wedding date due to Simpson's unexpected pregnancy. They are yet to set a new wedding date.

"It won't be this year," Simpson told talk show host Katie Couric. "I really want to do a big, fun wedding with my friends and family."