Clarification on unlawful possession of property with registered Will

My grandmother was widow and was working with a person for 37 years and lived in his house. I used to stay part time in his house after my birth. He made a will for his property to one other party and gives right to live to my grandmother untill her death. Other floors and shops given to other party.First floor of property where my grandmother lived with me and my parents. Donor gives me movable assets and give right to my grandmother to live untill her death. All other assets clearly mentioned in will to other party only first floor is in question.But other party claims that he is the owner of first floor though not mentioned a ownership to him.now the question is who is the owner of first floor? The will is registered.Donor had nor son nor wife nor other relatives in blood relation. We have a possession since 10 years.