What rights do I have on the shop after 40 years?

My grandfather divided the house into two parts. In the will, it is mentioned that half of the property is ours, and the house has 6 rooms and one shop. We have had possession of the shop for 40 years, and they have possession of 4 rooms, while we have possession of the remaining 2 rooms. Another brother wants to claim half of the property, but my father has had possession of the shop for 40 years. Can I claim the area of the shop for myself, as I have had possession of it for 40 years? Also, can I divide the rest of the property instead of the shop?