I hope this message finds you well, present circumstances excluded. I am a licensed attorney with over a decade of criminal law experience as well as civil litigation experience. It is a pleasure to assist you today.
First off, you have taken the correct steps up to this point. You are to be commended for that because most would be completely at a loss. The next step should involve local law enforcement. However, since they are unwilling to help, you may have to force the issue here. There are two ways to force the issue: (1) the local court system; and (2) the Attorney General's Office in Georgia.
Your first step needs to be that you formally attempt to file theft and conspiracy charges against your husband and his mother with the local sheriff office. If they are unwilling to cooperate, you can at least tell the court that you attempted to do so. You then need to file the same criminal charges with the court of competent jurisdiction in the county in which this is taking place. The court clerk may balk at first because there is no law enforcement involvement. You can then explain that they are unwilling to cooperate (It would be great and you should request a letter on the sheriff's letterhead that they are unwilling to file your charges). The court can take the charges that you file. You may also want to file a civil injunction request (emergency) with the same court against the mother in an attempt to seek an injunctive order which prevents her from blocking you from entering the premises to search for your items.
If, for some reason, the court is also unwilling to assist, you need to then speak with the Georgia AGs Office and request assistance. You need to also seek to file a Public Integrity complaint against the sheriff for his unwillingness to cooperate under the circumstances with the AG.
Document all of these steps as you progress forward.
Make no mistake that there is no surefire method to handle this issue if the authorities are unwilling to intervene. You can sue the mother civilly but that will take months, if not years, to resolve. In the short term, you need to proceed as directed above.
Let me know if you have any other questions or comments. Please also rate my answer positively (three or more stars) so I can receive credit for my response.
Best wishes going forward!