Hi Cindy, and welcome to planDisney!
I assume from your question that you are referring to the
Military Salute Tickets, which is an amazing value for Eligible Service Members and their families.
Because your sister-in-law was able to purchase tickets for you as an eligible service member or spouse, at least one of the tickets purchased is to be used by her at Walt Disney World Resort. The remaining tickets would then be assigned to the guests in the travel party.
"Eligible Service Members" are active or retired members of the US Military, including the National Guard, Reservists, the US Coast Guard, the US Space Force, the Commissioned Corps of the Public Health Service (PHS), and the Commissioned Corps of the National Oceanic and Atmospheric Administration (NOAA)." If he does have a valid military ID, I recommend visiting the military base they were initially purchased to explain what has happened, and they should be able to assist from that point. I hope this helps.
Please let us know if you have additional questions.
Have a magical
Walt Disney World Resort vacation.
Raquel