NPSP How to autocreate a relationship between two contacts in one household

I think you have two options:

Flow - lookup the other contact with the matching Account ID and create the relationship Spouse. (This will get into tricky territory when you bring children into Salesforce.)

OR

Create a custom field on the contact object called spouse and use the NPSP auto-create. (For simplicity sake you might want to only include this on one record type, but that could cause problems for two clergy who are married...)

PS: I was just about to manually create some automation to recreate what the NPSP auto-create does! Go NPSP with all these nice features.