Angel Sanctuary can best be described by the fact that for a good portion of it, you're watching it with your eyebrows raised. This is due to the fact that Angel Sanctuary is an incest story. To most of us who actually have sisters and are not reading nasty doujins, this is an absurd and revolting concept.
However, the concept here is taken very seriously and from a realistic perspective. Unlike most of the incest fanservice and occurances you'll find in anime, in Angel Sanctuary it is not assumed to be a normal engagement for the sake of some weird fantasy. In fact, how abnormal and wrong it is is fully addressed.
The story seems to be the manifesto of someone who has had incestuous feelings and has thought long and hard about it, about how despite it being wrong, it would make people that have those feelings happy to indulge in them.
The setting takes on a bastardized version of the war between Biblical angels and demons, the war against God, etc, still going on in present day Tokyo where a rebel Angel lies dormant in a seemingly human shell. It portrays the demons as the good guys, the angels as the bad guys. It says, "I know that for loving my sister, I belong in Hell, but I'd rather be there than in Heaven without my sister."
Angel Sanctuary is very short, at best a mediocre watch, at worst an advertisement for a manga I've never read. If you're curious about the ideas on incest, that's the most you'll get out of it. The intrigue of this peculiar topic alone is what got this anime a score of 5. The angel-demon setting could be pulled off better.