I can't speak for others, but for me I find younger characters and incest situations a major turn off. I'm not offended so much as grossed out by those things. Both are really prevalent in a lot of AVNs and NSFW games in general, so I suppose my perspective is unpopular (watch the down votes pour in...), but for me at least that is my perspective.
None of this is any indictment of Goodbye Eternity however. I want to make that clear. I am just about to download it, but was skimming through the comments to get a sense of whether this would be a game I'd like.