Maybe Sark and Sloane only think Irina is working for them; maybe that was her plan, to bring them down by making them think she's on their side. I still just want to believe that she really is trying to redeem herself; we have enough evil people on this show. I want her to be the one that truly does feel remorse, because God knows Sloane and Sark don't. Irina being evil would just feel like "evil overload".
Now that Sydney's lost Francie (even though she doesn't know it yet, I'm sure she will soon enough), her mom is the only close female in her life. If I were Sydney and I found out after everything else that Irina was just playing me, I'd probably kill myself. How much can the girl possibly take without it becoming inconceivable that she's not in a mental hospital? Sydney's tough, but she's not a superhero.