Ideas
This keeps happening to me, too! I fix the relationship, getting rid of both the 'romantic interest' sentiment and the relationship. After going to another household and playing it, it comes back again! Still trying to figure out if it's my mods or not--will update after I get to test vanilla.
Forgot to add, this is specifically happening to siblings--my own and EA Townies. In one save, every single Sim with a sibling had a romantic track.
-- Edit -- tested with absolutely no mods, and started a fresh save. Thought I'd check on the Harjos and...
Yeah. Of course, they're step-siblings. So far no other issues with say, aged up Amy Prescott and her sister Molly.
Step-siblings don't count as close-family members as far as I'm aware.
Do you have it among non-step relationships ?