He gives them back and runs.

He quickly rolls down his window and throws her underwear at her. Of course, Kate instantly recognized her brother. But before she is able to break from her complete horror, Sam is already driving away, running away from everyone he ever knew.

When Kate got home, she was so sickened by the event that she took a plan b. As a result, Kate never learned responsibilities, and Sam ended up being to one running from his own responsibilities.