A big spoiler for the Feed series by Mira Grant. Proceed at your own risk. (But the whole point of this post is that the spoiler should have been known.)
I started Feed for an online buddy read, and I was getting into it. Good world building, interesting characters, sad backstory, fun title pun. I posted about this in the group, and mentioned that I was just a little worried about there eventually being a gross romance. I thought I was being paranoid, but nonetheless my YA-dar was tripped.
The group leader responded — apologizing for not having known this and issued warnings — that they had just learned there is an incestuous element to the series, which is not shown on page in Feed but is revealed in later books. You could argue that it is not technically incest, since the characters, though brought up from birth as twins, are not blood-related. I don’t care.
I particularly don’t care after I went looking for reviews, surprised that people weren’t bugged by this. And I found one in which a reviewer gushes about the beautiful brother-sister relationship in Feed, and how it reminds her so much of her own brother. She even posted pictures of them growing up.
At first I thought this was gruesomely funny, but the more I think about it, the more furious I get. The author of Feed, whether intentionally or not, involved her readers in a taboo sexual situation without their knowledge or consent. That is ethically and morally wrong.
A Willful Woman... 
This makes me extremely wary of trying anything by this author. Consent is essential in romance, and that includes giving your readers the opportunity (knowledge) to consent or refuse.
Yes, well put.
though brought up from birth as twins
OH hell no. Nope, nope, nopity-nope-nope. They were raised together as siblings, from a VERY young age. I have too much history with adoption to ever see this as anything other than incestuous.
I can’t believe this didn’t bother more people!