Again. Apology for the late answer. Real life demanded my time.
I personally dont look at tags since I don't play these type of games for lewds but character / relationship building first followed by story. ive seen games where the tags are wrong, for example I played a game that had the incest tag but there isnt any in the game there is just a fan made patch for incest. so I always say it's best to look at reviews and the comments if something is important to you.
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.