Incest does play a part in the adult fantasies of both sexes, and you have described your fantasy very well. Your sister may well be enjoying your attention, but she may not appreciate quite how obsessive it has become.

Your sister deserves your respect and consideration - she has rights and feelings too. The more respect you show for your sister now, the better friends you will be in later life.

Fantasies become obsessive when we dwell on one theme too exclusively, and especially when we think of acting them out. They can actually reduce our enjoyment of the vast spectrum of other things out there, by training us to respond only to that one thing.

Rather than actively reinforcing your interest in your sister, why not look at some of the other kinds of fantasy out there? Enjoy the wider spectrum of human imagination, and focus your active sexual interest on someone else your sister's age!