Pseudo-incest. I remember watching a made-for-TV movie that broached this subject about twenty years ago (in a non-explicit format, of course). Is it okay to be attracted to your step-sibling? If you like it weird and kinky, the answer is yes.
At Smashwords: Too Intimate Sibs (and I bet Smashwords makes me change the cover)
or Amazon for your Kindle: Too Intimate Sibs
or Barnes & Noble for your Nook: Too Intimate Sibs