I used to watch (primary) softcore porn with my sister-in-law. When ever she and my brother would visit, we looked forward to Cinemax after dark or what ever tapes I had sitting around.
It was always quite amusing because while my brother would participate, he was always somewhat embarrassed by the whole thing! Especially when my sister-in-law would check to see if he was getting hard.
Edit: So no one else ever watched porn with their sister-in-law? Geez you guys are boring....