Hi!
I agree with
Jordanskene01 that Willi and Kati are still related (as are several characters more). Ocean has to work with Patreon´s puritan outlook/pressure on him, so we have the usual "bestest roommate 4eva" and other stupid walkarounds unless the patch is used.
So now for what is going on and how it should be without Patreon´s meddling: Willi and Leia are twins, Kati is the younger sister, Dylan and Helen are their parents, Kat is a cousin. Zoey and Stefan are neighbours and old friends of them.
One of the few things we can be certain when it comes to WIAB-SG crossovers is that Ayua is Miru´s kid. Here full name was stated in game, she is Ayua Zane, Kati is her aunt, she looks very much related to Miru and has the same low alcohol tolerance and general kickass outlook. So her Mom can be pretty much assuredly guessed. Her father should be Willi, unless Ocean wants to throw us a curveball.
Ayua has cousins (she tells us so, if you want to learn some special martial arts moves you have to marry either her or one of her cousins). I theorise that at least one, but not all, of those cousins is actually Ayua´s halfsister. Esp. in the case if in WIAB Willi and Leia fully reconcile, it is highly likely, considering their special relationship status, that Leia will at least once sport the "swallowed beach ball"
look, with Willi the reason for it.