That is so disgusting... but part of me wants to look it up.
I draw the line. If you are attracted to your sibling, there is something not right with you. The only case where there wouldn't be something wrong with you is if you were seperated early in life and then it would be somewhat normal if you were to meet later in life to have, not attraction... but it wouldn't be any different then a women (or man) you met off the street.
I know I read it somewhere, there is some biological, evolutionary, thingamajiggy that just makes you not sexually attracted to family members. So if you are some wires got crossed the wrong way somewhere along the line.