I have been thinking about this...
the effect may not the same from person to person, and perhaps not as dramatic. For example in a close relationship there might be periodic unexplained distancing which would defiantly effect another person. In a romantic relationship not being interested in sex would be a problem. Just being around someone who is always in a state of hyper vigilance would really cause stress on the others, I think in general there may be a tendency to feel hurt, alienated, or discouraged by being around someone with PTSD. I think being exposed on a regular basis to another persons trauma, without taking the proper precautions over a long period of time is going to be injured in some manor.