Hi all,
Thank you all for your replies and the reassurance to me that I'm somewhat normal. It means a lot.
I totally get what you're saying about the panties turning me on. I don't think it's because they're my sisters panties but it's because their sexy panties. They could belong to anyone.
I know if it came down to it I would never want to have sex with her for real.

I'm still not sure why I imagine Sarah locking me up and punishing me. Since my fantasy evolved to this level the orgasms are so good.
I have developed such a vivid and elaborate sequence of events in the fantasy that it almost feels like its real when I go there and close my eyes.
When I'm home alone and have the chance I will go down to the basement and play out my fantasy with her panties in one of the storage cages. All thats missing is Sarah looking in at me in disgust. At one time I "locked" myself in, wore only her panties and played the role of her prisoner for over an hour before I finally allowed myself to climax.

I almost want her to catch me in her panties down there for real. Will she react the way I imagine. Dangerous territory here I know.
Sorry for being so graphic here but its whats going on with me right now.
Thanks for reading.