YA half-brother asks step-sister to the prom?

[lila711]

Hi, I'm having a slight problem here...

I just moved out a newly YA son on the morning of his and his half-sisters prom.
Just before prom started, the teen half-sister got a call from her half-brother asking her to prom!
I accidentally accepted it and off they went.
He later asked her to go steady while at prom! Luckily I didn't accept it this time.

First of all: He's YA! He's a) not supposed to go to prom, and b) too old to enter into a relationship with her.
He's also her half-brother! I though the game was supposed to prevent inter-family relationships?!

I'm doing the 100 baby challenge and if every kid has the possibility of ending up in a relationship with their half-siblings I think I'm gonna go crazy!
Any help would be much appreciated.

Edit: I changed step to half. They are half brother and sister, not step.

[Figwit]

A similar thing happened in my game though it was with the teen girl's YA nephew.

During the prom, they just had 'friends' or 'acquaintances' in their relationship panel but afterwards they went back to being niece and nephew.  Once the teen girl grew to YA though, she developed a love interest for her nephew.  He is married though and (I think) quite unaware.