They were both about to be married when strange things began to occur.
They both hear a low whistle in the dead of night.
They were both afraid of their step-father and his wild animals that roamed freely on the grounds.
Helen is in a similar position as Julia was in several ways.
They are sisters.
They are or were living with Dr. Roylett.
They fear what was about to happen (or, in the case of the dead sister, what did happen).
They heard a mysterious whistling in the night.
They sleep in a room with a false bell rope.