This site's
incest tag definition mentions that blood relations are typical, but that implies that they are not a requirement for the tag. Tag stays. If you think there should be a stricter definition of the incest tag, that's something to take up with the site's admins/mods in a thread in offtopic, not in an individual game's thread.
Also, your claim that "several" other sites do would need support. The other major site like this one, Fuck95, also does not separate incest and stepcest/adoptedcest, and even more explicitly includes those as part of the incest tag:
View attachment 63455
So the tag applies there, too.
It's fine if you disagree with the assessment that the incest tag should include stepcest and adoptedcest. I even agree with you that they should be separated. But until they are, there's no use getting into arguments with other users, mods, or uploaders about it in individual game threads.