I'm having the same issue and it is incredibly frustrating. It seems to be counting Engaged sims as "related" thus not letting you preset your sims as engaged in CAS. I started a couple, got them engaged, and then tried to start the scenario but it won't let me because they don't meet the "sims must not be related" condition for the scenario. I also tried starting them from scratch in CAS as a brand new start, and it wouldn't let me select the engaged option. So I went with household member, but then they start out as just acquaintances, which kind of ruins the entire point of the scenario and renders it useless.
