For sims that are attracted to each other that you don't want them to pursue their attraction further, there is an option in game where they can choose to remain just friends and not become lovers. It's a dialogue interaction.
Sadly this has proven not to work for a long time, they'll eventually flirt again, which I actually like because it says "friend" so i can see them break that IRL. Exactly the same feature saying "family" would give us both options to either "cool down romance" or "turn off/turn on romance indefinitely".
Have you tried to give the turn offs? It might help with the attraction thing so that they don't feel tempted to flirt. Also check if you have things in the house that might get them in a flirty mood, like flirty paintings, the ice bucket, or cookies, etc.
