Hi, Carol -
First, please accept my condolences on the recent loss of your Mother. Mom's are the most special people in the world.
If she left no Will, then yes you can prepare a simple Affidavit of Heirship on your Mother's Estate. If the subject properties were also owned by your Father you will need either his Probate Papers (Will, etc) or to include his history in the Affidavit as well.
You will need to file the Affidavit and certified copies of any Probate Papers in the Official Public Records of the County(ies) the properties are located in, at the same time ordering a set of certified copies of those documents to submit to Antero's Division Order Department.
Antero will then issue new Division Orders to you and your sister. Once you have signed those and returned them to Antero, they will release any royalties held in suspense during the interim and you will each begin to receive your monthly checks.
I can send you an example or two of an Affidavit of Heirship if you would like or you can download a form for one from the internet or maybe ask Antero to send you a blank form.
Hope this helps -
Charles Emery Tooke III
Certified Professional Landman
Fort Worth, Texas