Sister

Indeed, as suggested by the first commentor, there are many factors to inform and qualify this claim... im not a fan of **, its not my cup of tea, but far be it from me to judge anyone on their preferences. We're biological creatures... yes, evolved ones with the mental capacity for choice to bypass these kinds of behaviors that even in the animal world have things in place to prevent ** (for the most part)... but maybe its our concsious knowledge of the "forbidden" that makes ** so alluring to so many...
If she is an attractive woman, then i wouldnt be too ashamed... she's good looking, you're a healthy male, it's biology... it all depends on your psychology in the matter, whether you would actually really do something... from your need to confess, it seems you're aware that its deemed "wrong", but really its your choice.
But thinkin about it? Eh, i guess it's understandable to a point, just depends what particular detail about the fantasy is actually doing the turning on.
*MistaJ

I ** my sister every week...we are in our thirties now and both married...it's our secret.

Did you grow up with her, or is she a new addition to your family? How old are you two?