Based on how you've described things: I think your sister-in-law is confused. I worked outside the U.S. for over eight years. I never had any such form or explanatory.

I can only think of two instances were anything comparable might exist. First, if one works outside the U.S. for roughly a year any monies made under $70,000 is not taxable. One has to declare this is the case on a form. Two, for resident aliens (those who are working for or who have obtained Permanent Resident status) if they are out of the U.S. for over a year (or perhaps a lesser period, I cant recall) and plan on returing to the U.S. to live: they have to declare this intent so the government knows they still a pursuing Permanent Residency.