From the many-to-one documentation http://nhforge.org/doc/nh/en/index.html#mapping-declaration-manytoone (you're doing this on a one-to-many, but the same property-ref rules apply):

The property-ref attribute should only be used for mapping legacy data where a foreign key refers to a unique key of the associated table other than the primary key.

(emphasis is mine)

If you are going to use RelatedPersonId as a property-ref, there should be a unique constraint on that property, which would prevent you from inserting two people who both have RelatedPersonId == 5. I'm guessing this isn't what you actually want to do, though. Just a shot in the dark, I think you probably want a many-to-one on Person named RelatedPerson, and also a one-to-many on Person named Addresses that simply points to Person.Id, not using property-ref. Then you would get the related person addresses via person1.RelatedPerson.Addresses.

Test case