Most of the incest fan community considers stepcest, or any other non-blood-related "fake incest" to be a slap in the face. The incest tag(under the current tag system) has to be applied even when the incest is "fake", so the fans tend to get pissed/annoyed when they see the tag, only to find out there's no real incest. We get that you can't control the tag, so the hate is generally aimed at the site, and the perpetually teased tags 2.0 system.
If you can make them blood related(at least an option), and have it make sense, I think you'd make a huge portion of your potential playerbase very happy.