Incest is wrong? That's news to me!
While I wouldn't partake in it (I bear no attraction to any of my family), the only reason I can see for the taboo is the narrow minded misconceptions derived from years of belief in the "fact" that the powers that be are against any form of relationship which cannot "bear legitimate fruit." See "mercy killing of the disabled" for more.
This actually links back into Lucky Star (not through canon, of course) where we have the double whammy of everyone's favourite wincest pairing contained within a religious family. Pretty much every fanmade work involving Kagami/Tsukasa and their coming out sees them disowned in some form.
Now, with everything linked in nicely, I'd quite like to say this- do not purchase the English release of the Lucky Star manga. Why? Well, while we have the obvious plus of supporting Kagami and his works, the simple truth is this; Robert Place Napton RUINED it.
He is, of course, the editor who saw fit to ignore blatantly jarring sentences and boring lines of could-be laugh-out-loud humour, whilst not even having the grace to read over the middle parts of the volume. While we could place the blame at the translator's door (there's more than one), they are almost exclusively Japanese, and can't be fully expected to churn out perfect English.
Heck, just looking at the cover itself will tell you the state of operations here; couldn't care less. The front has clear, ugly signs of resizing and pixellation, and don't even get me stated on how half-arse the back is.
I'd whole-heartedly recommend buying the original Japanese works, forgoing the English Bandai releases for two simple reasons; a. Online scanlators clearly put three times as much effort into this, and they don't even get paid for it, and b. Bandai's just releasing it for the sake of releasing it while waiting for a quick buck to roll in.
I'm sure Kagami wouldn't appreciate more money being siphoned off of his hard-earned royalties than necessary.