I think it's mainly the taboo that governs how I feel on this, it does feel wrong due to the fact that you share blood. Then again, a mate of mine recently started getting feelings for his soon to be step-sister, and that felt odd to, and there is no shared blood at all. I think the problem lies in that you are altering the role of what it is to be a sister, a cousin etc. I also don't agree with it for medical reasons, surely it cannot be very healthy genetically due to increased chances of inheriting a disease both of your parents have.
i thought only people from alabama liked their cousins