I enter the estate under Sanctuary, nobody sees me at all, but anytime I steal something under Sanctuary spell then the Flaming Fists suddenly appear and tell Branwen she committed a crime.
How they saw her stealing or even approach to talk to her under Sanctuary is beyond me. Looks like a bug
Edit: Apparently even if they "catch" you stealing under Sanctuary they cannot do anything. They start to roam searching the thief. But since the thief is under Sanctuary they cannot nor see him nor attack him. So you can just walk out of Oberan's estate and leave them to search. Just don't attack them as it will break Sanctuary.