Am I entitled to the plot if my mother dies and father is remarried?

When my brother died, the cemetery donated 3 plots to my mother and father. My Parents divorced and my mother remarried. Now she has passed away, and my Step father has remarried. He has not placed a tombstone on her plot and refuses to let me put one there. The plots are in my biological father and mother's name. My biological father signed the plots over to a trust that was wrote up by my mother. What rights do I have with the plots so that I can put down a tombstone on her plot?