Live-in Relationship

My mother (50yrs) wants to do a second relationship with a man (57yrs). She doesn't prefer to do a proper marriage instead, take this relationship as a Live-in Relationship. Both the parties, "the man" and "my mom" have 2 adult kids each and, both the couples earned well. So, my problem is, my dead father left his inheritance for her wife (my mom) and their children, and I as an elder son doesn't want to lose cause this relationship. So my question is, what document should I prepare so that in future if they decided to end this relationship, none of them dare to claim other's assets. Thank you and please reply me back.