...but to me incest is a story dynamic that has to be presented with care in order to keep the attraction both believable and positive...
Everything you said Cryssoberyl is very accurate, but condensing to this sentence because I feel it needs the most emphasis.
Society has stuck a boding and inescapable connotation on the word "incest", so much so that many hear the word and automatically assume '%sibling raped %sibling'. And completely and understandably so. As a survival tactic, it makes sense to try and get into people's heads that having children with your blood relation is bad. Not only this, but the nuclear household just naturally develops these days in "developed" nations that inherently make it not only nonsensical from almost every perspective, but also rather distasteful to even consider falling in love with a sibling.
So from a practical sense, healthy incest is likely only achievable in stories. For that very reason, the intrinsic dynamic is incredibly unstable and fragile. The story has to be pure (in a connective sense) and well-ironned at its roots in order to capture an audience. This one did not do enough of that. Even the idea of them not doing anything to look any different from the other is forgivable and surprisingly workable from a story's sense...but what broke the hinges was the way they so nonchalantly accepted the idea they could taken by someone else...**as if someone else could legitimately tell them apart and fell in love with one of them, they would simply just have to reciprocate**.
That just naturally makes the relationship become a "because we have no other option" scenario, which is not a bad start in many cases; but as the main focal axis? Bad.