When your mom was alive she would own the copyright to her art work from the moment she created it. I suspect that she never registered a copyright for her art work with the U.S. Copyright Office
. But registering a copyright is not necessary for ownership
. It is necessary to enforce a copyright in court (but more on that in a moment). Now that she is deceased, ownership of the copyright to the art work would be passed to and be owned by her estate. Thus, as the administrator of her estate you essentially have the right to request that anyone using the art work without your permission and/or without compensation should be stopped.
Now it is possible that while your mom was alive she granted permission to others to use her artwork by some type of agreement. If that is the case, you wouldn't have the ability to prevent those users from continued use depending on the terms of that agreement. You'll have to do a little questioning of the current users on exactly how they came to use your mom's artwork and under what terms. But for any future users, you certainly have the right as owner of the copyright to grant or deny permission for use, and to request royalty payments under a licensing agreement.
To stop any actual infringement of the copyright (use without permission) you would have to commence a lawsuit against the infringer. And before you can bring a case in court to enforce a copyright, you have to register your copyright with the U.S. Copyright Office
. So, I would recommend that you register a copyright for your morn's art work, and as administrator of her estate you can do that.
I hope this helps.