We are never told she's a full on lesbian and there's no character that only has a dark path either. There's about 4? sexual paths for every character. Making Cassie the only one with less would be weird. Especially when she has been so talked about through the story.
We don't KNOW she's a Gold Star lesbian, but when the first chapter of Season 2 comes out, you will see why I think it's likely. Even if she is bi, honestly, the idea of Cassie in ANY kind of a "romantic" light side relationship, much less with her brother, would be out of character for her.
But Cassie
isn't a "love interest." She's Guy's sister. It's not the same thing at all. In the real world, incest is a corrupt thing, and this game may bow to porn logic in places, but it does take place in a more grounded world than any of the family harem games.
But hey, tell me this; what is the completely non-corrupt, romantic love scenario you would write for a brother and sister fucking which wouldn't break the tone OF THIS GAME?
Because as a fairly long term supporter of this game, I can't think of one way which wouldn't feel out of place. And most of this game's paying subscribers are doing so, as much as anything, for the writing. The art started out, well, kinda bad, but that's been replaced with the new Chapter one, and it has improved to very good, but not enough to play the game just for that. There's no music, and basically no animations. The real availability of choice is there, and that's great, but in and of itself, who cares? Even the pretty extreme dark path routes are only worth playing because they are well written.
The thing that sets this game apart is the writing, and there is no reasonable way to include a romantic incest relationship which wouldn't break the whole thing. I don't want that. Neon's subscribers, for the most part, don't want that. We DON'T just play this game to jerk off. There are plenty of those games out there, and if that's what you want, have at it, but that's not, and never has been, THIS game.
But hey, maybe Neon will come up with something. That would be fine, but if it doesn't feel as well written as the rest of the game, he shouldn't shoehorn it in.