The Otoboku series is often put forward as a prime example of a sub-genre that first gained popularity about eight years ago... the 'trap protagonist in a girl's school' type. However, there are a number of aspects that make this series a bit unique... or at least make them feel unique.
One is their protagonists... despite their differences in personality and upbringing, all three protagonists end up taking a similar role in the common route, even aside from the 'Elder' issue. To be straight about it, the Taishou-era 'oneesama' concept, where girls in an isolated environment form half-romantic relationships with older girls, is the biggest influence on these games. The odd irony of a trap playing the same role as one of those 'oneesama' characters often had me smiling in humor and exasperation, but for some strange reason, Takaya Aya seems to make it work every time.
Another aspect of these games that is unique is the rather blunt way each of the games portrays the 'old wealthy families' of Japan's attitude toward their females, as well as the attitude of the 'new rich' toward them as well. Despite skepticism on the part of some when I've mentioned this in the past, it has to be noted that the wealthy in Japan still frequently use arranged marriages to form connections and incorporate capable individuals into the top of family-run businesses. The former kazoku families, in particular, obsessively arrange the marriages of their children due to traditions going back over a thousand years (including a tradition of ignoring commoner's 'common sense'). While the most recent game is a bit lighter on that issue, you'll still hit it in several of the paths and even in the common route (though the common route is more like a gentle manipulation to try to stick two people together).
Last of all, in every one of these games, at some point, gossip gets out of hand and causes at least some of the characters to suffer. This is actually a common thing in games based in girl's schools, but the way Takaya handles it is generally more interesting and emphasizes the isolated nature of the gender-restricted environment.
All of this comes together to create a game that has a deliberate atmosphere of 'isolation from societal norms', one of Takaya's favorite themes in his games (every single one of this games does this, regardless of who he is working for at the time). For this reason, the series has a rather unique 'taste' to it that isn't quite matched by any of the other similar games I've played.