This strikes me as the right answer to the wrong question. If you go out to the Dog N Suds and valet your car, and while you're enjoying your dog and your suds, some third party steals your car, do you worry about whether the thief stole your car from the DNS? No! You worry that the thief stole your car from you! So who cares if the SIL cannot legally steal from the daughter as a matter of criminal law? He stole your gun from you, period. Report it stolen and report who you think the thief is. I'm not sure I see an added benefit of suing. It will cost a lot more than the value of the gun, which is what your damages are, and if you know the SN on property that was stolen and who stole it, the police should not have much trouble finding it. But if you do sue, in GA you have to sue the defendant in the county where he resides.