How to void a gift deed or claim for a share

Respected lawyers, I am writing on behalf of my mother. The property in question is a two stored building in Bidhan Road, Siliguri, West Bengal. My mother has two more sisters. My grandfather who passed away around 2005, had made a gift deed of the said property to his second daughter around 2001-2003 which was not told to my mother. My mother has two elder sisters. We got to know about it because of a big incident that took place in 2006. Since then the family is not in good terms over this matter that my mother was never informed of the gift deed. The property is not yet registered to the new name and we will like to know if it is at all possible to get the share or void the gift deed. There is more to the story. Sharing just to see if this will help because this was not just a think planned between father and the second daughter but our tenant was also involved. These tenants have a cycle store occupying the ground floor for over 20-25 years. They are not good tenants in respect to behaviour and they are continuing to pay 200-400 contesting they are old tenants. These tenants want to occupy the building and is their main motive. So what they did is they became friend with the second daughter and lended her some 2 lakhs for her personal use. They have got her signature on a 10-20 rupees bond paper saying that she will register the shop in their name. The money was taken before the gift deed was made. Now there is a case running in the siliguri court because the tenants want the whole building for 2 lakhs or not sure if they are willing to pay more than 10 lakhs. But in no ways my mother wants to sell the property. The problem is the gift deed is made on her sisters name by her father which she did not know at all. Please help me to understand how can we claim for the share. Also, the other thing I have noticed is that the gift deed was made by the advocate the tenants use. I saw the gift deed and it has the advocates signature, the second daughter's signature, second daughter's husband's signature and my grandfathers. It has cost 50 thousand to make that gift deed and my grandfather did not have that kind of money then. So what the tenants have done is, they paid for the gift deed and did all formalities because at that time they were friends with the second daughter planning it will be easier to get the property transferred in their name. My grandfather also had no idea that his second daughter had signed on a bond saying she will sell the property to these tenants. When my grandfather found this wrong doing he went under depression and fell down and died of brain hemorrhage. The tenants have also broken down and removed all traces that there was a pukka stair case because the registration was not done per their plan. with the help of local corporator there we have fixed a kaccha staircase to the building, which can fall anytime. This was all their plan to get the property. Both the tenant and the second daughter tried to act smart and both are fighting the case and I see it as a fraud by both. After that the second sister does not share anything about what is happening in the court with my mother. My mother was also asked to file a petition saying she wants the share so that they will not loose the house. We don't know what is happening because the petition was files by the second sisters lawyer. I don't know if they are calling mom for hearing and the message is not told to my mother. We don't know if in such case the case is dismissed or what should we do. With all these background of our problem, I would need your guidance to understand what should my mother do to get her share or if she will ever be able to claim and win the case. Please help and provide your guidance.