Vel, take it as I'm saying it:
My first husband was a pathological liar. From my experience with his family, it is a genetic disorder. Remember something, when you are closely involved with/married to someone, you are not just with them, you are involved with their family.
If you think at least one member of his family has sticky fingers and problems with the truth, guaranteed more than one member has that problem and blood IS thicker than water, one will lie and the other will swear to it and s/o will probably side with his family over you (particularly his mother, that's a given).
If nothing else, you will need permanently need to start inventorying your stuff before they come over, you will need to remain in the house at all times with them, you will need to lock interior doors when they are coming over and I'll guarantee you he's going to give you shit about not trusting his family.