However, my cat and my parents' cat are working out a bit of a turf thing right now and having a foster in the house (especially longer than the initial "just for the holiday weekend") is kind of heightening the Territory Wars *just a tad*.

You can sometimes calm the territory wars a bit by putting Feliway diffusers in the rooms they hang out in most often! We can always tell when ours are out; there is suddenly far more hissing and swatting.