Yes, in the WIAB thread there are semi-regular jokes about Willi and Leia "bestest roommates 4eva"
and the like, since the patched and unpatched version of old WiaB were even clearer on this. But even new WiaB is rather open about it. When Willi tells Kati that he can always get under her skin even today, she answers him that as long as he does not get under Leia´s skirt all is good. Well, if the teaser pics we got for ch.3 are to believed, on the "reconciliation with Leia too" path he even keeps to that. Leia does not wear a skirt then...
And we also see that Willi sends a very enjoying look down Leia´s dressneck at breakfast, Helen is spying on the two what they do when alone, there is still something between the two.
While only some bits of old WiaB´s storyline will be transfered to new WIAB, there was a scene in the old version where Willi and Leia are in bed together. They only go down on each other, because Leia is unprotected. But they keep it like that not because she is fertile, but Leia says herself, she would fuck him right here, but she does not know currently if he will be staying in Wollust.
So I tihnk in any of the WiaB storylines were Willi and Leia reconcil we can be rather sure Ayua will have halfsibling(s).