Well, the uncommonness of the sexual sibling relationship is a given, my main point was that among the possible love interests for Willi (or the SG MC) there are persons which are much more likely to get into a kind of relationship with Willi than others. Which is why I think that progeny is far more likely between Willi and Miru, Leia, Zoey, Daphne (mentioned too often already, rather sure something might happen here later on) and Dana than any of the other female LIs.
Here in our case of Kati there is not only the mainly antagonistic life experiences, their sibling status, but also Kati leaning more towards lesbianism, which make such a relationship very unlikely. Only specific player intervention can change this.
I know that there are "Hate fucks", but considering the situation between some women and Willi, there cannot be many.
