Provisions to cancel fake mutation will,title deed

Hello Sirs/ Madams. My father belongs to family of 5 children. My grandfather built house in India after partition. 4 of his children left this house gradually/ were asked to leave by him and finally only one was left with grandfather in 1985. That child (my chacha) made fake will favoring the property to himself and his wife. He did mutation in 1998, while my grandfather died in 1987. He has been living in property ever since with his wife and kids. 4 other original heirs never asked for their share, but now some of them require funds for themselves & are requesting for amicable partition. However family of the son who got fake will made (he has died recently) claim to be legitimate owners of property on basis of fake will, mutation and also a title deed that seems fake as well (showing ownership in chacha & wife name). How do we get mutation cancelled? How much time do such cases take to resolve. Property is in prime location in Delhi and worth upwards of INR 5 Cr in our estimate.