Because the preface summarizes the important connection between your story and that of your relative, you should write the stories first. Now that you have a working draft, you can write a longer paragraph that:

Directly tells the reader about how the stories connect
Includes specific details about your life, and also about relatives life
Develops an artistic example as metaphor for the lesson

For this last item, think about what you have learned about yourself through his project, what conclusions you've drawn about the similarities between yourself and your relative. Select either a song lyric, a famous quote, a line from a poem, a literary allusion, a film scene, or other artistic endeavor that illustrates the lesson. Be creative in this last item, and look for a significant (but not obvious or cliched) example.