mourning glory,

I can understand your fantasies, but I think the advice that the others are giving you is very useful. I will not give you anymore, but I will relate my own experiences as a sister and perhaps it will help you see this situation from your sister's eyes. She probably feels much as I did many years ago.

My brother and I were very close growing up. We were best friends and played together all the time. As innocents we sometimes explored each other sexually, just touching and looking and such, but once we both learned about sex, it stopped immediately. In fact, he was the one who explained the facts of life to me. It would have been devastating for me and ruined my relationship with him, if he had tried anything sexual after I knew about sex. He didn't, and we are very close to this day.
I was the older one, but only by a year, and he was much bigger than I. I know he has always thought me pretty, and I think he is a fine-looking man. That's as far as it will ever go.