What makes the movie work is the writing and acting. The dialogue covers the basics of what you'd expect to hear in a "relationship drama," but it does so with such humor and intelligence that any preachiness is generally minute, even if there is still a slight sense of self-indulgence. Equal credit is also owed to the actors, who deliver the dialogue with the necessary amount of potency and charm. Adam Brody deserves extra mention for finally getting the chance to prove himself as a leading man; lets hope his post-OC career serves him well.
If there is a flaw in the writing, it's that sometimes the off-center qualities of the characters are a little too similar, since they all seem to share the exact same philosophical banter. Especially jarring is the younger sister, who spouts out such witty insights that you can almost hear the actress becoming confused as she says them. I'm all for having intelligent characters, but throwing in a "brainy kid" is such a cliché, and in this case, a frustratingly unnecessary one.
This, however, is hardly an issue compared to what happens later in the film. I won't spoil it, but it's so predictable that it wouldn't matter if I did. That's actually part of what makes it so annoying, since it really shouldn't have been predictable; nothing leading up to these two particular sequences feels deserving of what eventually transpires. It's as if the studio was desperate to add a more Hollywood angle to the relationships of these three characters, regardless of the change never being justified. In the end, it's not enough to kill the positive qualities of the film, but it certainly takes its toll.