Solution for incorrect will

My father and mother passed away leaving behind a House Property. My father had written a Will and had registered the same. As per the will the property is to be shared by me and my brother. My brother passed away leaving behind him his wife and two children. My sister in law and son are occupying a portion of the house presently and other portion is vacant. I have decided to occupy that portion now. Unfortunately that portion is in damaged condition. As per the Will , my father had offered 2 rooms in the ground floor and 2 rooms in the first floor for my brother. The remaining 4 rooms in the ground floor including kitchen and toilet is written in my favour. The portion on top of the 4 rooms which is an open terrace had been written in favour of my brother. This I am afraid had been wrongly written in the will Now , to occupy my portion , I need to repair or even demolish my portion. My sister in law claims the open terrace belongs to her and is not willing for demolition stating she will move to the court. I am unable to occupy my portion and seek your advice on course of action