One of the more popular Arrow theories making the rounds is that Thea knows that her brother is the man under the hood but has been lying about it to him.
The speculation is that Malcolm Merlin told her during one of the father/daughter training sessions.
In a recent interview with TVLine.com, the show’s EP Marc Guggenheim effectively shot this theory down immediately.
“I’ll tell you, because I don’t think it’s a spoiler: Thea does not know that Oliver is the Arrow,” Guggenheim confirmed. “We sort of played that trick with Moira — the idea that ever since the end of Season 1, Moira knew that Oliver was the Arrow — and to have Thea be in possession of that knowledge and not be telling Oliver would be repeating ourselves a bit, which we try hard to avoid doing. That said, the reason Malcolm doesn’t tell Thea Oliver’s secret is because that is Oliver’s secret to tell. The thing about Malcolm is we’ve always tried to write him as “the word’s most principled bad guy.” He has a very distinct code of honor. Even when he destroyed the city at the end of Season 1, he considered it to be for very noble, altruistic purposes. He was “burning the village to save it.” So where Malcolm is concerned, he doesn’t act like your typical villain. He believes that he’s doing the right thing by Thea, and he also believes that Oliver is like a son to him. Being like a son to him, he’s not going to spoiler Oliver’s secret.”