Hi, i had the same problem as you, except it was the other way around. I was the sister and it was my brother who confessed his feelings to me (i have a thread up on here if you want to look at it to get more insight into my situation. I don't update it anymore b/c the majority of posters here were just being mean instead of helpful). Anyways, me and my brother are attempting a romantic relationship right now. I'm not going to go through the excrutiating details but it took a LOT of resisting, debating, frustration, mind games, etc. to get us to where we are now. Where we are now is not any easier mind you but we are both very much in love. There's no going back now for us.

Having said all that I don't recommend you tell your sister your feelings. Just keep it to yourself. It will open up a can of worms and will probably make your sister feel very confused, pressured, guilty or even disgusted with you. You have no idea how she will react and do you really want to risk your relationship with her? Even though me and my brother are very happy in our relationship there will always ALWAYS be a constant worry/anxiety about other people finding out about us and us being forced to separate (not to mention the shame we will feel). Believe me, as much as i love my brother now, i think i was happier not knowing that he loved me. At least i never had to worry about losing him.

P.S I hope you're not just poking fun at my situation b/c your situation sounds TOO similar to how mine was.