Perhaps you're only adopted on Patreon?
And i think that relations between step-relations is legally defined as incest in at least some jurisdictions.
I'm fairly sure step relations have no legal rammifications anywhere in the world. Incest is only prohibited due to gene recession causing genetic mutations that can lead to problematic symptoms for those born from it which in turn constitutes a form of child abuse.
I would say it's more of a
Social stigma then a legal issue persay, society as a whole tends to find the concept of family members getting it on as taboo and since a relative by marriage is the next closest thing to a relative by blood society will often take the simple route and tar them both with the same brush so to speak.
In layman's terms the avarage humans thought process is tantamount to (incest is bad, your parents are married, that makes you related therefore you can't be together)
That said the incest tag could simply be for anyone wanting to patch said game so the adopted part becomes irrelevant or perhaps in corrupting said daughter there might be some daddy+daughter/mommy+daughter scenes.