I've read about this sort of thing happening. I think it was in some case law crap or something. Basically someone, usually younger, moves in with someone, usually older and there are often medical concerns or the potential for medical concerns. Eventually you end up with more people connected to the younger person either moving in too or becoming extremely close to the original person. The original person either ends up writing that person into the lease or will or whatever or something occurs where the original person ends up disappearing, dying, or ending up for an extended period in a hospital or home to recover and the person who moved in takes over.
Different states have different laws when it comes to squatters but the longer someone has been living in a place, the harder it is to get them out. The eviction process in ND itself allows for no less than 30 full days after notice before an actual allowable action can be taken. And that's just the rental side of things. So-called significant others that are together until something bad happens are a whole other kettle of fish.