Proposing RelatedPerson.relationship become a BackboneElement with a role, relation, and period field.  In our system, related persons have a both a type/role and a relation.

For example, types/roles are Emergency Contact, Guarantor, Family Member, Power of Attorney.  Relation examples are Father, Mother, Brother, Sister, Spouse, Self, etc.

The Mother could be an Emergency Contact, Guarantor, Family Member and the Father could be an Emergency Contact, Family Member, and Power of Attorney.  

On a Read, we dedup Mother from each of the types/roles (Emergency Contact, Guarantor, Family Member) and return once.

For a Write and Update, it is very problematic to figure out how to set the role and relation correctly.