Will Execution

We are two sisters and both are married. My mother expires on july 09. She had made a will 3 days before her death from cancer. In which she had given entire property to me and money to my elder sister. As i had take care of my mother on the time of her treatment she had given me extra then my sister. Now as per the will the property is executed on my name. now i want to sale out that property but my sister and her husband is threatning me that they challenge the will if i will not give them 50% of the proprty. I had already taken the token money of that land from the purchser.Please guide what i have to do.