On the Red Wedding plot card it says. "Target opponent chooses 1 lord and 1 lady. You then kill one of those characters and the other claims 2 power." I translate that to mean any Lord or Lady controlled by anybody. Am I right or are there choices restricted to their own characters?

Since it doesn't specifically say "that they control" or any other such restriction, the only thing the choice is limited to is two separate characters, one with the Lord trait, the other with the Lady trait.

A beast of a plot if you can time it right.