I am also experiencing this bug as of approximately 2 months ago.
It started off with the father and his young adult daughter being each other's romantic interest (even though both are happily married to other sims). They would both get very sad if they saw the other flirting with their partner.
I used the "modify_relationship" cheat to eliminate the romance bar in their relationship, but their relationship status went from "romantic interest" to "just good friends" and/or "it's complicated".
As if this wasn't alarming enough, about a week ago, the father and his child son also became each other's romantic interests! This is especially strange as obviously children can't be interacted with romantically, nor can children interact romantically with others.
I've tried to destroy and rebuild the incestuous relationships using cheats, but the rebuild cheat doesn't seem to work for me (I could be entering it wrong to be fair, but I've had no luck with it so far).
I've also contacted an EA adviser about this and they confirmed that this is a bug and advised me to post on http://forum.ea.com/eaforum/categories/show/257.page which has led me to here.
I do hope this gets fixed soon, I don't want my sims to go full-blown Lannister style on me 😕mileysad:
