Incest is only wrong in situations where the family is involved. The only reason incest is a problem is because of the way our society and life is structured--we have family units. Other species probably have incest all the time due to the way they live, ie: their young leave as soon as possible, and there aren't any other siblings around while they still need care.
So I would say that neither situation is wrong, until the other side (respectively) is considered. The first is OK until it is revealed that the two were brother/sister, and the second is OK until it is revealed that the two were raised as brother/sister.
To me, neither is a problem, only because I've never had a sister, so there isn't any disgust that I can derive from the thought, and the second is OK because it isn't biological.