I am only going to say this once here, and it will apply to all degrees of incest.
I have only two problems with it. The first is the potential offspring, the second is that barring extraordinary circumstances it is very likely to fuck you up mentally and emotionally. Extraordinary circumstances such as "I didn't know I had a sister, let alone that it was her".
The familial bond is a very powerful thing, and most such bonds in today's world cannot involve sexual relations without being affected for the worse. However, not all blood siblings share that bond, due to distance or what have you. Take out the social aspect and children that may result if they're not careful, and who does it really harm? I'm not suggesting that the social aspect is a minor one at all; simply that it's not always there.
I have no moral objection to love between two people who don't socially share a familial bond, regardless of their actual genetic relationship. My worldview allows for two lovers who are just like any other couple other than a genetic closeness. And I would wish them all the best. I would draw the line at offspring, but apart from that, go nuts. Just be careful how you publicise it, for obvious reasons.
So, to actually answer the OP: Go for it.
And now, I shall equip my flame shield, for I am clearly a sick bastard who should be shot for valuing love and happiness over social norms.