The property is earned by your father or he inherited from his father? This is a very important point to know to decide what action you have to take.
1. If the property is purchased by your father our of his own earnings, he is the owner and he can decide the way he wanted and can give to any one he wanted. But he has to make a Will in which he can mention the names of the persons to whom he wanted to give his property and in what proportion. If he writes his self earned property to your stepmother she will be the legal owner of the property after the demise of your father.
2. If the property is inherited by your father from his ancestors, he can't write the Will as he likes and after his death, you will also be eligible for sharing the property with the children of your father with his second wife. If the second marriage is not registered and if the marriage is not legal they can't claim the part in that.
Please contact a lawyer and explain to him the entire history. Then show him the papers what you have with you. If you have a known lawyer, you can go to him. He will definitely guide you properly. You should give clarity to him the points which are mentioned below.
1. How your father obtained that property?
2. Is the property documents contain the name of your mother also?
3, Is the second marriage legal and registered?
Once he gets the clarity, he will advise you and give you a road map to follow.