Is the step-incest mentioned in Content Warnings something the player will be able to actively partake in? And if so is it a one-off thing or something persuable?

Okay, so! Bit of a spoiler, but it's talked about on Patreon and Tumblr, that in the planned Book 2, that Starling Knight, the town Mortician and Doctor, is available to romance. If you're playing the Knight Origin, in which the player was adopted into a prestigious academic family, this means that Starling is technically their adopted uncle/aunt even though they've never met before. It's more pseudo-incest than Step, tbf. But, yeah, if Knight MC want's to pursue a romance with Starling Knight, it's something that will be discussed and a bit of a struggle to navigate for them. You can romance Knight with a different MC, or just not romance them! Hope this helps!