I was watching a porno movie (while stretching a few muscles) a few months back. All was well until I saw a chick that looked like my sister. I stopped what I was doing (because I felt uncomfortable) and tried to continue, but couldn't because all I could think about was my sister, so I skipped to the next scene (thanks to DVD chapter technology.)
My answer is... NO because I would have my mother/sister in mind and wouldn't feel comfortable.