The value set applied by IPS for patient.contact.relationship is https://terminology.hl7.org/3.0.0/ValueSet-v3-PersonalRelationshipRoleType.html and the strength applied is Required. 

While this value set has considerably more detail than the V2 value set that FHIR sets as extensible (http://hl7.org/fhir/valueset-patient-contactrelationship.html). 

I'm not sure where this falls in terms of extensible rules (if it's legal profiling change to modify to a different value set when there is a broader value in the original v2 value set that could be applied - like next of kin or emergency contact or even unknown). But I think the more consistent approach that would be less likely to confuse implementers would be to keep the extensible v2 VS on the parent element and creating slicing on coding to show implementers how the IPS V3 value set could be supplied as an extensibly allowed coding (or as an additional coding to provide more granular details to the proper v2 code).