For SP to face federal charges for lying to an FBI agent, I believe she would have had to lie directly to that agent. We don't know if the FBI ever interviewed her. Casey Anthony was interviewed by the FBI, but I don't think she was charged with, or convicted of, lying to the federal government.
The fox article I linked above says she's remained in contact with local LE and the FBI, but idk how involved the FBI has been in terms of interviewing her. Didn't she work with an FBI sketch artist to create the sketch of the 2 kidnappers? Either way, I personally don't think she would be charged with lying to an FBI agent, unless LE decided they were really going to make an example out of her. I think the FBI is more concerned with charging white collar criminals who lie to them (i.e. - George Papadopoulos) as opposed to supermoms, but that's just my two cents.