I don't know if you aware of, but the crowdfunding platforms that host these games and from which new releases are leaked to this site - their rules prohibit the sexualization of incest. Many games whose authors ignored or tried to get around this rule, were removed from the platforms, and their devs lost the opportunity to receive financial support for their work. For example, this game was removed from Patreon in the middle of last year.
Therefore, developers change the plots of their games and the relationships between characters so as not to violate these rules. At the same time, they try to create a sense of taboo relationships between characters, tension because of this, while remaining within the rules and making their characters speak as vaguely as possible on these topics.
However, if you directly ask the developer about this on his Discord, firstly people there will show you the rules of his server, which prohibits discussing the family relationships of the characters, and secondly they will point you to a pinned post, where the author says that Leia and Katie are Willie's old friends. Sapienti sat.