What do you do when a relatives child steals something from your home?

My husband has a granddaughter. This past week-end we had a baby shower at my house. The granddaughter has a half brother, who does not live with her who attended the party. He stole my son's $500 Blackberry phone. He is about 13 years old. If I confront the Mom and Dad about him stealing the phone, I feel they will be defensive and hurt. I am afraid if I start trouble over the phone, the Mom will get angry and not let us see our granddaughter. At the same time, I am boiling mad that this child stole from my son in our own home. I don't think a $500 phone is worth risking losing being able to see our granddaughter, but I am very angry. What do I do?????????