Your mother more than likely willed her half to your father and in the event of him dying before her, the house would have been divided by the four of you. If the house was in joint names the family home automatically goes to your father regardless of what the will states. Your father would be very wise to make a will. When somebody dies without a will, the family have to come along and prove that they are entitled etc. Is your father mentally aware of things? He would need medical certs etc. to prove that he knows what he is saying when he does make his will. You would be very wise to discuss the situation with a solicitor.