How to talk with a family member politely about their child stealing from my child?

I recently had company/family over at my home for a small get together... My son has a Nintendo DSi and a clear case with 12 video games. If you are familiar with these games they can cost anywhere from $14.99 to $35.99, long story short, we noticed that the clear case with all 12 video games was missing. We backtracked and searched our house high and low... I even dug through our trash to make sure that it wasn't thrown out on accident. My husband and I have our speculations that it was stolen by an 8 year-old cousin. How do I confront my Aunt with this news and how do I get the satisfaction that they did there jobs as parents to make absolutely sure that my 8 year-old cousin did not steal the games? We are talking approximately $360 dollars in missing videos games. (Side Note: This is not the first time that this 8 year-old cousin has stolen something before!)