This sort of thing is as old as the hills [1].  If a child does not grow
up with a sibling or parent, there is *often* an extremely strong
attraction if they meet in later life.  It's often said that "opposites
attract" but it's not true.  All the evidence suggests that people
are attracted to people who are like them (both in physical appearance
and otherwise).  Who would be closer than a sibling or parent / child?

Equally there is clearly some sort of mechanism at work that prevents
brothers and sisters from becoming attracted.  (The first case quoted
in the article is highly unusual in this respect).  I have read that
the early kibbutzim in Israel brought up their children communally and
hoped that they would in due course pair off and start a new generation.
Instead what happened was that the kids basically felt about each other
as brother and sister and simply didn't fancy each other.

I am not making any judgements, just stating facts.

[1] http://en.wikipedia.org/wiki/Oedipus