Property dispute between me(son) and my sister and mother

2 years ago

I am a Hindu male living in West Bengal. My father passed away 2 years ago, leaving me(the son), my mother and my sister. He had bought our house with his own money in 1982 and a flat in Kolkata. Before his death, he had created a will clearly stating that the flat will be inherited by my sister and the house will pass onto my mother. He stated in the will that after my mother passed away, I the son will inherit the house. There is no mention of my sister inheriting any part of the house as she will be given the flat. It was also not mentioned in the will if my mother will have the right to dispose of the flat to anyone.
I have been living in this house since it was bought and am now currently living with my wife and daughters. All these years, I and my wife took care of my parents, paid for my father's cancer treatments and bore all their expenses. My sister convinced my mother recently to move to the flat she inherited in Kolkata. After a month my mother moved to Kolkata, she and my sister have sent us a notice that they will be selling off their shares of the house, although my sister doesn't have any share of the house in the will.
Can they sell off the house to a third party?