I think it's all about concentration, where your head is at. We can't really do two things at once, can only concentrate on one thing at a time, people who are "multi-tasking" successfully are actually sequentially tasking :-) Everyone dissociates; I think of all the times I've driven to/from work and can't remember the drive or when I've gotten "lost" reading or "brooding" to music :-) and hours have passed, etc. So, what your sister-in-law experienced from you could be either one, your just paying more attention to what you were doing than what she was saying or you dissociating. I still remember the time I was driving home from therapy and totally missed my exit on the freeway and almost panicked when I realized it, had to figure out another way home from "scratch" and quickly (as the other exits I could choose were coming up and I needed to review which would be "best" and get in the correct lane at the right time during rush hour).