Do you let sims with common ancestors that the game no longer considers related be romantic?

I would let siblings be romantic if the unmodded game would allow it. The only thing that bothers me about in-family romantic gestures is the common behaviour of in-laws to autonomously flirt with eachother; and mostly because I ordinarily do not wish it to happen, since it can easily destroy their relations with their partners and the other's partner, and any toddler that catches the interactions, etc etc... The game is very open and closed at the same time.