Let me get this straight... Arquen takes part in murdering Lucien Lachance, a fellow Dark Brotherhood member, and she winds up being given control of his Sanctuary in the end?

She allowed herself to be deceived, and she allowed it to get to the point that she murdered an innocent, and an innocent that also happened to be a fellow member as well. Even if you excuse her because she was "tricked," giving her control of a sanctuary afterwards, and the sanctuary previously controlled by the member she murdered in cold blood no less... wtf?