Family Land Dispute

Hi, My dad expired recently and left me with a step-mother, her son (step-brother) and a married sister (step-sister). In short, I am the only son of my dad and mom. We have 18.5 acres of agricultural land (14 on my dad's name and 4.5 on my step-mother's name). Right now, my step-mother is hiding the land registration documents and putting up an act (no on except me is aware of this). How can I get my rightful share of 9.25 acres from the total 18.5 acres under this circumstances. Also, is it possible that my step-mother can claim ownership of this complete 18.5 acres and not give me any rights on it forever