Second Marriage and step sister rights in property

Hello Sir, My father is passed away recently (Oct-2017) and his second wife is asking for some compensation which i'm ready to give on humanitarian basis. before giving anything to her i want to make sure that should not legally affect me on long term. what kind of agreement can i make for removing their rights from the property?. here is some basic information about my family. First wife(my mother) and we are 3 children's all are over 29 age. Second wife is having one daughter she is around 22 age. all are alive. What i know about Hindu act is Second marriage is Void and the daughter is having rights on ancestor property. let me know if my knowledge is not correct. and also let me know if they go to court. we dont have any ancestor property in my father name but we have ancestor property in my mother name. how it affects us? as my father second marriage was some what secret and my family knows about it but not bothered much so i want to keep this as secret as possible (thinking about the Non-disclosure agreement also) let me know can Non-disclosure agreement works in this case. your help is much appreciated.