I want to know Ineritance rules.

Hi, My name is Harsh. There is a house in which we live, which was bought by my Grandfather by selling properties at our village. This property is now in the name of my Grandmother. There are 3 sons of my grandfather and grandmother. My Father is youngest of all. The property has 3 different houses on it. In 1 house the eldest Son lives with her wife. In another lives 2nd son with his wife and grandmother, he has 2 daughters which are married. In last house my father, my mother, me and my younger sister live. All property document are with the 2nd son (middle son/ my uncle) after the death of my grandfather. No other brother has seen any property documents or know about it. We have been living on this property for last 25+ year. All the brothers pay different light bills and tax bills in the name of my grandmother. We were living on this property till now on the basis of trust, but recently I met a person who came to see the property in order to buy it. This means my uncle is trying to sell the property. Can he sell the property with the signatures of grandmother without informing us? My grandmother is illiterate and can sign any document without reading. We are unaware of any kind of documents of this property. Can we challenge our inheritance and right over property? What kind of steps should we take now in order to pertain our house? Any other suggestions or advise? Shall we consult any lawyer or take any legal step as precaution to this kind of activity? Thanking you in anticipation for your help and advise. I can also provide any further information if needed.