Incest is okay with some folks apparently. Lusting after your own child is quite okay. When someone says "if she were not my daughter I would probably date her" sounds lustful to me . You too?

I think that it all really depends on the relationship that the people have that determines how things turn out.  Just with any other couple, a family relationship can be based on everything ranging from platonic love to animal lust/carnage and everything in between.  For some people, it's a way of life, for others it's a fetish fantasy and for others it's a point of revulsion.

As for your question if "if she werent my (child/sibling/relative) I'd date (that person)," sounds lustful, it'd really depend on what the speaker meant.  If they meant that the person was interesting and that they wanted to have a deeper, more intimate (but not specifically physical) relationship with the person, then no, it doesnt sound lustful.  Instead, if the person meant it in a kind of "I'd like to pin that person against the wall" way, then yea, very lustful.

It all just depends on _how_ the person is saying _what_ they're saying.  Context is everything.

Yes, context is everything but unfortunately the word "incest" does refer to a specifically sexual desire, not just a "deeper, more intimate relationship". However the desire need not be as overtly lustful as in "pin that person against the wall", which implies control more than intimacy. It is still highly illegal in most societies and countries, though, and not just if forced.

There have been few reported cases as far as I know, but the poet Lord Byron (1788 - 1824) is said to have had an affair with his half-sister.