At this point I don't believe that she had always been such a law abiding citizen. She just never got caught. Remember, she joined the county sheriff's office relatively late, at least 31/ 32. She may have been a party child with a relatively dark side before that then put on a show to get the new job. Her first husband used drugs, perhaps she did as well? Maybe she made extra money smuggling contraband to prisoners under the inept eyes of elected sheriffs?
I think her Miss Perfect and Innocent image was for show. I mean who would suspect anything of the waddling grandmother aged simple woman? I halfway expect her waddling to straighten out like Keyser Soze's limp did.