With Heavy Rain just around the corner, the critical elements of the games industry (or at least Penny Arcade and Kotaku) have begun to ponder: has the video-game evolved to the point where it's able to represent and explore human relationships? Have we reached the "baby boomer" era where the gamers who started playing in the 1980s are looking for games which they can better relate to, now they have 2.5 kids and a mortgage?
In truth, I'm not sure. Ever since the first anthromorphised sprite bounced onto the screen, game developers have been looking for emotional hooks to motivate the player, beyond the basic attraction of high-score tables and level clocking.
With virtually every aspect of human civilisation revolving around social relationships, it's not surprising that relatives and lovers soon became integrated into game plots: Mario jumped over barrels to rescue his girlfriend while Robotron saw you rescuing an entire family - though this was something of a rarity, as the majority of games (Mario, Double Dragon, Mikie, etc) focused on the rescuing of girlfriends. Though perhaps this isn't too surprising, given the audience demographic was heavily focused on young males - and the majority of game developers were also young males! Playing the hero and getting the girl was perhaps a fantasy in more ways than one on both sides of the system...
Moving on, all of these early games had a common feature: the person/people you were struggling to protect were MacGuffins: their only purpose was to provide the player with motivation. You could replace Princess Peach with a potato, for all the difference it makes to the gameplay. There's a simple reason for this lack of interaction: early video games simply didn't have the computing power to do anything else - and with most players lacking the skills/money to finish these games, putting together a coherent story or ending was generally not worth the effort.
These days though, we have the ability to throw polygons and AI routines at our MacGuffins, so that they can be an integral part of both the plot and the gameplay. Sometimes they even become the game, as in The Sims.
So: is there a general trend towards more mature use of "relationships" in video games? Sadly, I'm not all that convinced. Mario is still chasing after his princess and the majority of video games are heavily focused on escapism - saving the galaxy, fighting the evil power, solving the mystery - and of course, getting the girl.
Of course, there are games which do make much greater use of relationship constructs. The Silent Hill series positions you as a father seeking his daughter, a husband seeking his wife - and (much more unusually) a daughter seeking her father. Ico saw you trying to take a friend to safety (to grossly oversimplify; the relationship between the two certainly wasn't presented as having any sexual overtones); Shadow of the Colossus tasked you with saving your lover through the sacrifice of innocent creatures. The Project Zero/Fatal Frame series involves rescuing a brother/sister/fiance/friend. Farenheit drew you into interactions with your brother and your lover and now Heavy Rain looks to do similar with your children. Bioshock 2 asks you to defend your "sisters" (though to me, this seems more like a natural progression of the first game's themes, rather than a concious attempt to weave emotional attachments into the game's plot and mechanics).
However, these games are definitely the exception, rather than the norm - and it's also notable that the relationship theme is generally tied into a series, implying that it's individual designers/developers are exploring relationships, rather than the games industry as a whole. Also, for the most part, these games aren't particularly mainstream: they're generally adored by video game critics (and may have sold well within their demographic - e.g. survival horror), but the sales are a fraction of those for games with more traditional, MacGuffin style objectives.
It's certainly a good thing that these themes are being explored and intelligently applied but it's debatable whether or not they'll ever go mainstream. After all, anyone can grab a gun and save the damsel in distress; it's much harder to interact with someone and build a relationship. That way, reality lies...