Our food co-op site uses relationships, e.g. Household Member. We have a custom relationship type: Proxy Shopper. We're interested in making Proxy Shopper conditional so that no individual can get Proxy Shopper status unless both individuals are Members of the same Household.
Jerry and Willow are Members of the Ortega Household. Willow owns a co-op membership with which she can shop at our store, and Jerry does not own such a membership. However, Willow has designated Jerry as a Proxy Shopper so he'll be able to shop on her behalf.
Willow's mother Sabrina, however, is a member of the Sturm household (not Ortega), so Sabrina should not be a proxy shopper for Willow, even though Willow wants to send her mother to the co-op.
In short, we're looking for a way (possibly through an extension) to condition the Proxy Shopper relationship type upon membership in the same Household.
Worth noting that while belonging to the same Household should be required for setting a Proxy Shopper relationship, Household membership should not predict a Proxy Shopper relationship. In other words, no Household Member should become a Proxy Shopper unless someone sets that relationship via the admin interface.
What's the best way to do this -- beyond just telling backend users that they shouldn't make anyone a Proxy Shopper unless that individual belongs to the same Household as the second individual?