i'm planning to set up a town with a family structure that means that people who aren't marked as relations by the game are going to be raised as siblings - how do i disallow a sim from romancing a specific other sim so that I don't have story progression pairing these psuedo-siblings up?

I think you could just place them in the same caste and use the Caste: Disallow settings. Say your caste was called Family, you'd create it and set it to Manual, so you could add the sims you wanted manually. Then within the caste settings for Family:

Flirt: Disallow Caste > Family
Romance: Disallow Partner Caste > Family

The idea here is that SP could not push any sim within the Family caste to have a flirt or romance with any other sim in that caste. You could create multiple ones, for example Family A and Family B, if necessary.

Keep in mind that the game itself will allow sims to autonomously flirt with each other if they're not related or age-blocked. I've seen this in my own game—even though SP is disallowing any romantic progression on various sims, they end up flirting in the park in front of a whole crowd just because they're friends and one has the Flirty trait. Adding Woohooer should fix this: on default settings, it checks SP's settings and blocks any romantic interactions that are disallowed within SP.

Or you could forbid one of them to bathe. Although that might have other implications that I'm just not thinking of at the moment...