A different theory (spoilers)

I watched this last night, without knowing anything at all about the film or the plot. I admit, I was doing other things while it was on (dishes, laundry) and so might have missed key plot points.

But my take at the end was that the whole thing had been a set-up. The "big reveal" was that the two seemingly fragile siblings had actually cold-heartedly set the whole thing into motion, and duped "Declan" into being a tool for neutralizing their mother. I didn't think this until the last scene, when brother and sister are both suddenly UNfragile and steely-eyed. But it explains so much: the purposefully inept bomb plant in London through to the manipulative passion of the kiss at the quayside -- "please help me, I'm just a damsel in distress," but suddenly she's revealed as tough as nails and crying few tears, either for her mother's death or the agent's death.