Lat u know after a while and even after trying it . And tell if it was successfull or not . Till d time u can even try my idea (well d above one) Hope It Helps !!
They will once you complete the Life Dreams & Legacies Quest, that is when the live like the normal Sims3/4 games
I have siblings dat is sisters n 1 is toddler n 1 is baby. D moment I bought d baby d relation bar had 3 people dat is mom n dad(acquaintances) n sis(sibling) so it just remains as sibling cos toddlers can't perform actions wid babys n I'm gonna do it as tod-preteen n bab-tod n see what they can do cos I saw a cheat on youtube in which d bar had sister n brother