Actually a foster or step sibling would have potentially more of the love/hate rivalry. If we take Willi and Leia as related (prior THE redaction) with all of them foster children because Dylan's sterile but Helen wanted kids, there could EASILY be the kinds of dynamics we've already seen, with Willi and Leia ganging up on Katie, Dylan not knowing how to dad properly and setting them up in pit fights, Helen finding Willi attractive, it's a whole lot more plausible, still has the taboos, but elevates Leia and Willi's plot.