Distributing the estate in a C.O.P Marriage

My father past away in 2008 he was married in c.o.p. I have 4 sisters and me as the only son. What happens to the house as they want to put the house on to my mother's name and they say she owns half the house. What does shariah say about this and how do I go about in sorting this out. The house was bonded with my late father and mothers name, now she is the executor so how does this work?