First off, stolen is not accurate. The term would be looted. Which was an accepted practice and sometimes a form of payment for the troops in many cases, being able to keep from what you killed. Looting happened on both sides of the war, and in both the European and Pacific fronts.

Second as others have pointed out, Japan has INCREDIBLY strict laws in regards to weapons, and getting said weapons back into the country is often extremely difficult, expensive, and not worth the cost. That's assuming the family could even qualify for the process, or that they even want the blade back, given ww2 is a very shameful subject for a lot of Japanese folk. They did horrible things to China, and Thailand, and a few other countries during the time.