How are Jimmy Swaggart and Elvis Presley cousins?

When Elvis and Jimmy's cousin Jerry Lee Lewis were both on the bill at Louisiana Hayride in Shreveport, Louisiana, Jimmy Swaggart (from Ferriday, LA, just like his cousins Jerry Lee Lewis and Mickey Gilley) met with his cousin Jerry Lee and Elvis backstage after the show. The only photo of this meeting that exists belongs to Jerry Lee's sister, Gail Lewis.