More seriously, I have nothing against it, I do play games like that sometimes, but I don't really see how it would work well here. I guess it could be somehow believable with Rion being the little sister, and Maddie the big sister, but the story and plot are not like MILFY CITY or other similar games with just a daily routine of a family life, which is obviously made with incest in mind. Here the story is not really made for that. (and won't)
Anyway, I have no such plan at the moment, as it is a bit far from the story I want to tell and from the "natural" relationship approach I want to do.
Now, if a lot of Patrons request it, doing an optional patch for that in the future is not something impossible to work out, as it doesn't require many changes and the storyline could still be the same without interfering with the people who will still want to play with the main original setup (friends).
So, it's a no, not at the moment. But it's not a definitive no.
Unlike Harem which is an ultimate NO here, because it will destroy all the principles and structure of this game.