Son's inheritance of fathers house

We are christians(middle-class) family in karnataka.My father separated(not divorced) from us 10 years ago. Can i challenge in court for a share of his self earned property(his house), even when he has secretly written a will(his house) favouring another person unknown to us?. Can the will be challenged before or after his demise ?. His house is in haveri in which i live along with my mother, where as he lives in harihar for the past 10 years .