Your query cannot be answered for want of following facts:
1. What is your age?
2. What is the age of your sister?
3. What is the age of your step brother?
4. What are all the assets in the name of your father?
5. What documents you executed relinquishing your share in favour of your step mother?
6. Whatever given in writing by your step mother is signed by your stem mother or not?
7.How much money she had given you in order prove her honesty?
8. In what form the money is given either cash or by cheque?
9. How everything has been transferred? What documents have been executed in favour of your step mother?
10. When she did all that.
11. You can get your share if you take appropriate steps immediately by giving complete details of the facts to the lawyer you hire.