Unless your cousin-in-law says, "Yes, I have the Luger; I was keeping it for you and here it is" you don't have a lot to go on. You can't even prove it was ever there, let alone that a specific person took it or when; if a Luger turned up you couldn't even prove it was the same one since you didn't record the serial number and markings.
Sorry, but I think the gun is long gone.
jim
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