I had the same problem. I went into the room with the sister, tried to attack her, attacked civilians, attacked soldiers, fast traveled to Lunden and back, did a world event and even closed and restarted the game. None of that worked. Eventually though, I went back into the room wit the sister and then exited the animus, checked a flower pot, talked to Shaun and then re-entered the animus. I walked closer and the scene finally started. I don't know if you HAVE to talk to check the potted flower and talk to Shaun, but just trying exiting and re-entering the animus while in or near the room with the sister.