It is about 4 non blood related siblings with evil step parents. So evil, that the 4 siblings want theses step parents to die. The rest of the movie is the kids not knowing what to do with the bodies, and annoying relatives and neighbors wondering where their parents are. Some sequences are very funny, but some are tiresome.
There's the hot older sister, who has brainless sex with some guy in a truck every night, the oldest brother, the one who kills the parents; you know, the responsible one. There's the death obsessed 10 year old, who's shtick gets boring after 5 minutes, and there's the ADORABLE youngest girl. She has lots of cute one liners about you know, parents dying and stuff like that.
The chemistry is great between the four of them. That makes the movie. They work very well together and are really believable.
Bottom line: definitely has its moments, and if you like these kind of movies, then you should go see it. But regarding black comedies, don't expect Very Bad Things (the KING of Black humor movies)