When I turned 18, I registered to vote. Just before I turned 18, I registered with the Selective Service. I did not get drafted. In fact I went to college and then voluntarily joined the Air Force.
It sounds like your sister's, boyfriend's, cousin's, room-mate may have not registered with selective service and is getting a kind reminder that he needs to submit his name.
He might want to fill out the card. He is not being drafted. The country just likes to know who is available.
There was an article posted last week,I believe,that reported some idiot having a friend shoot him in the leg because he thought it would keep him from being drafted. HarHar! :o)