Family House Case

Hi, I was born in a polygamous family which I am the first son of the family, my father had a land which i developed and built a house and accommodated my step mum and the children, when my father died some years back, we started having issues about the ownership of the house, here in my tradition, the first son of the family inherits his fathers house which i did, as time goes on the first son of my step mum (my step brother) keep threatening and agitating for the house to be shared equally after I have giving them some rooms out of the building, claiming that the land which I developed belongs to our late dad that the building should be shared equally, After all the fights and quarrels, my step brother, after realizing that he is going on a wrong direction, now came to me seeking for peace, but am not comfortable with his hidden agenda. i told him that am ready for peace but i will need us to reach an agreement before signing any peace accord. please I want an undertaking document which I can present to him to sign to accept that the property belongs to me, to avoid future occurrence of the case.