For a writer, it's an easy way to raise the stakes -- these characters suddenly have a whole new, deeper connection you didn't even know about.
Why It Ruins Stories:
This twist tends to come up more often in a series (either TV or movie franchises), and usually it's something thrown in at the last minute (honestly, would Lucas have played up the sexual tension between Luke and Leia so much if he had known from the start that they were brother and sister? For his benefit, let's assume not). And worse, it's usually something they come back to repeatedly.
For TV writers, for example, it's become a go-to twist to drop in at the end of an episode. And in shows that are built on twists, this becomes an issue when they've pulled this one out of the bag every six episodes and by the fourth season everybody is related. We're not exaggerating, by the way -- just look at Lost. Claire and Jack turn out to be half-siblings. Daniel Faraday is also half-siblings with Penny. That creepy Horace guy turns out to be creepy Ethan's dad. The only two Asian characters introduced after the first season turn out to be father and son.
Thankfully they dropped their planned "they're brothers and brothers" reveal.
In fact, one of the biggest shocks of the series comes when John Locke discovers that a man who he thought was his dad was just trying to scam him out of a kidney -- they had so many surprise relatives in this universe that the twists came from the fact that two characters weren't related. Heroes fell into this pattern, too -- Hayden Panettiere's character is Nathan Petrelli's daughter, then Meredith Gordon's daughter, then Flint's niece (and if you don't know who any of these people are because you stopped watching after the first season, that's a good sign).
The last season involved an evil super-carnival ... seriously.