Rights of a widow on her in-law's property.

My sister's husband passed away last week due to critical illness. They have two daughters, one 14 yrs of age and the other one 7 yrs. My sister's mother-in-law is planning to give all the property to her daughter who is already well settled in the US. We requested her to at-least think about the kids future but she is adamant and not willing to listen. We are planning to take the legal route and wondering what chances we have to get justice. My sister's mother in-law and father-in-law were both doctors. My sister's father-in-law passed away before my sister's marriage. Now my sister's mother in-law is saying that the property is all her's and not inherited. How can she claim that the entire property is her's when her husband was also a doctor and was a bread earner for their family? How can we know what portion of the property is earned by her husband? The property belonging to her husband should at-least be divided equally among my brother-in-law and his sister right? How can we fight back to get justice to my sister?