I need legal help with ancestral property partition.

So here's my case. My grandfather is 100 years old. And he is now not well, can't talk, or walk or remember things property. Sometimes he behaves like child, like crying and all. While he was healthy, sound, walking and talking(when he was around 80 years), he had written a WILL will equally divides his property among his 7 children(2 son, 5 Daughters), he had allocated his wealth in such a way that there would be no dispute.(House for those who don't have a home, and property and money for others equally.) Now that he is bedridden, one of his son and daughter, changed the majority of the WILL secretly to their name. They took grandfather to registrar in pretext of taking him to doctor and changed majority of the property, home and all to their names. Leaving very small amount to the rest for name sake. lets say out of 4 crore worth wealth, 3.9 crore was changed to their name and rest 10 lac to others. Since grandfather is old and ill, he doesn't know what's going on. He is unwell and doesn't have any memory. Grandfather is still alive, but bedridden. Nobody knew this change of WIll until recently. Since the above son keeps record of all the papers. Now I was wondering if there is any scope for this case and what are the documents and evidence I require to strengthen this case.