"Multiple developers found their visual novels have been banned from Steam as Valve tackles erotic games that feature problematic or possibly illegal content, such as school settings or incest. "Dec 10, 2018
School settings are absolutely an issue (lots of games coding high school as "college" to try to get around the rule, but Steam has been banning those as well), and I think when they refer to incest, they're almost exclusively referring to parent/child incest in situations where the child's age is ambiguous or can be set by the player (which is what happened to
A Petal Among Thorns when it was denied for publishing on Steam).
Steam really doesn't have an problem with incest in general. Their rules have no mention of it, and I've spoken with a handful of other devs who have published on there without issue.
One key concern for a lot of devs is abiding by Patreon rules, since they will absolutely ding you for posting versions of your game elsewhere that include incest. However, Patreon has had no problem with
Bare Witness and it appears that cousins having sex isn't something that is against their rules. If that changes in the future, we've already got a backup plan for amending Sadie's relationship to MC.