Forbidden Love

His eldest sister (half-sister) just got married, and his dad looked really upset. I thought that was strange, because he was acting like a jealous sim. I thought "don't tell me he had the hots for his stepdaughter!?!" I checked his profile from her, and sure enough he was listed as "romantic interest". Ew, that's creepy.

But I guess it's ok, because they're not related by blood. But hey, he married her mom when she was a child (when they moved from xbox to my pc), so he's practically her dad.

Would the romance stay in place when the child aged up to a teen?
I aged up a pair of teenage twins, and one had a girlfriend. When he aged up to an adult, their pink romance meter went away.

(It's just as well. I would have had them break up anyway, so he could find an adult girlfriend or wife. I was just surprised they did it on their own.)

FWIW, the romance meter doesn't bother me, because they don't do anything romantic. It's just a romance bar. I just rp that he has a crush on her, and pretend that her feelings are not the same.

One concern I've had was if they would start having romance interactions when he ages up to a teen, because then they might start having romance interactions. If that happens, though, I could always have them ask to just be friends.

I have a household where the son brought home his new girlfriend (she was part of another of my households that I wanted to link together by marriage). Upon meeting, the dad had romantic and flirty feelings towards the new girlfriend which was not what I wanted at all and wasn't really making a whole lot of sense. He isn't a romantic sim by design and there was nothing in the room or conversations taking place that would have created a romantic mood. Turns out, one of the active Neighborhood Action Plans was "Free Love" and that was creating the overly amorous feelings. Perhaps that's what's happening here? Once I changed that, the sims went back to normal.

First time I ever used a cheat to adjust the settings in my game was to correct that. I went through every neighborhood in every world and adjusted that and a few other troublesome ones out of the list as I find some make the game absolutely unplayable. I hate the Neighborhood Action Plans!!

As for children developing crushes on people - that does happen. But it lacks certain components that crushes at older ages have. It makes them more on the innocent side than anything else and the child grows out of them. An adult may find them cute or endearing but will not want to act on them or take them further. In the Sims, "love" lacks much of the nuances it has in reality as it all seems to be related to eros (romance related) not philia (friendship related). So even a generic hug will feed into the romance meter. At least - that's my opinion - I could be wrong.