In general, see Alabama Code Sections
7-9A-609 and
7-9A-607. The repo man can, within the confines of the law, repossess the car provided there is no breach of the peace.
It sounds like the repo man either believes that your mother knows where the car is and that she is deliberately hiding it, or that he can intimidate her into finding out where the car is such that he can repossess it. If you believe his actions rise to the level of a breach of the peace, you can contact a lawyer about whether the violation would support a civil action.
If the car is not recovered (or even if it is recovered and there is an arrearage after it is auctioned or sold) and a judgment is taken against your mother-in-law (and brother-in-law) for the full amount of the debt, the creditor will likely try to collect against any non-exempt assets. A primary residence is often exempt from execution under state law; she can verify which of her assets may be subject to garnishment or execution with a local lawyer.