I STILL hate the idea of a sister and this AM I came up with another reason why.
Presumably, Irina had to have some clue that the sister was really the one from the prophecy. In that case, she has allowed Syd to be tortured, interogated, imprisoned etc. all while protecting the sister. It makes me sick and very angry. To sacrifice one daughter in the name of another all for some prophecy...grrr!
No Sloane likely heard something about his daughter from Conrad that monk in Tibet. I'm betting that he made the assumption Conrad meant Syd because he is unaware Irina ever had another daughter.
Conrad must also have revealed something to Sloane about Irina's purpose
Sister = bad