Regarding inheritance of father's property

We are Hindu. The flat I am currently living in was owned by my father. The flat was legally transferred to my name by my father on 31-7-2004. I have share certificate on which it is written that all members of housing society have approved unanimously that I own the flat. I have submitted following stamp paper documents to society- 1. Indemnity Bond, 2. Declaration by both me and my father, 3. Undertaking by me I have been living in this flat for 19 years now. My father is still alive. Now, My sisters are demanding a share in the said flat to my father stating that the flat was once owned by my father. They are saying that they will file a case in the court if they do not get a share in the said flat. My question is: 1. Can they file a case to get a share in the said flat? 2. Do my sisters deserve a share in the said flat?