Dear Bruce,
you need to establish ownership for that property to be able stop selling.
You may go to the cemetery administration and find who owns plots. It sounds like your stepmother is the owner - you just need to confirm that. You may also want to inform the cemetery administration (in writing!) that you are objecting the sale.
The second thing you might want to do - go to the probate court in the county where your father lived at the time he passed away and find out if his will was probated or the ownership on his estate was transfered any other way.
If the will was not probated - you need to find the Will and find out why it was not probated.
You also need to be prepared to answer the question - why after your father died you did not requested the ownership for the plot.
The answer is an advisory opinion (not legal or financial advice). After accepting the answer bookmark it - you may return to this page later for reference or to ask for clarification. Appreciate your positive feedback.