Family dispute over house property

We had a family property ( land) equally divided between three bothers given by the grandfather. It was decided to build houses as floor wise by among three brothers as the land size was less. It was decided that myself (second brother) will made the first floor then the second floor will be made by youngest one and thereafter the eldest one. I have made the floor but the construction was not finished due to financial issue. after that the youngest brother has made his house in second floor and after 7 years he demolished the wall of first floor that i have made and made a staircase from over there and extended the roof of his balcony upto 10 ft from the front and thus covered the first floor house. And everything was done without any intimation to the me and eldest brother as both of us are residing outside our parent state. now there is a violation of mutual agreement and demolishing of wall was not acceptable. how to resolve the issue legally ? kindly advise