Again. Apology for the late answer. Real life demanded my time.
Well. Without the tags, it's pretty hard to search for games that includes your fetishes. That said. I do agree that the tags alone isn't enough. Therefore I also look at the game descriptions and reviews (in those rare cases there are some) before I download a game. But neither the tags, nor the game description warned me that this game only have a few scenes that can't be rated PG-12.
Regarding the incest tag in the case you describe, I do believe the incest tag is right. I primarily uses the tag to search for games the includes my fetishes. I do believe that people who are into incest does the same, and therefore the incest tag is correct event that the incest is a fan made patch.