Share of Property between step siblings.

My father have one son from his first marriage with his first wife and she dies when the son was small. He got married again for taking care of the first child. And have three children with her, one son and two daughters. She never discriminate between the four children. What are the property rights of the four children in the father's property when both my father and mother were dead. I am confused because someone told me that my step brother have half right of the property becouse he is the son of firth wife. And the other half is divided into three children of the second wife in 1/3 part . Please guide me with the correct provision of property rights when the father have two marriages.