Shit, I wish I understood your question better before I responded. Chances are I wouldn't have.
WHAT INDIVIDUAL you are attracted to is completely different than WHAT SEX you are attracted to. If you are attracted to your sister/brother, that is an issue of circumstances (there is no reason, much less evidence to believe any differently). If you are attracted to men/women, that is (at least, by many indications) an issue of your being.