Here is the division according to Chinese Inheritance Law

According to Chinese Inheritance Law, as I learned in college:

The key is to how to divide the assets of your biological father and biological monther.

when your biological mother died, if she has no will, her half of the marital asset went to your father. 
Half went to you and your sister.

when your father passed away, if he has no will, half go to his surviving wife, half goes to you and your sister.

So, your step mother technically can get a quarter of the asset of your biological parents.
You and your sister can get 3/4.

The key is to collect evidence on your biological parents' asset at different point in time.

Good luck.