Taste the Blood of Dracula

Three English gentlemen - Hargood, Paxton and Secker - have formed a circle ostensibly devoted to charitable work but in reality they indulge themselves in brothels. One night they are intrigued by a young man who bursts into the brothel and is immediately tended to after snapping his fingers, despite the brothelkeeper's objections. The gentlemen are informed that he is Courtley, who was disinherited for celebrating a Black Mass.
Hoping for more intense pleasures, Hargood meets Courtley outside the brothel. The younger man takes the three to the Cafe Royal and promises them experiences they will never forget but insists that they go to see Weller and purchase from him Dracula's ring, cloak and dried blood. Having done so, the three meet with Courtley at an abandoned Church for a ceremony during which he puts the dried blood into goblets and mixes it with drops of his own blood, telling the men to drink. They refuse, so he drinks the blood himself, screams and falls to the ground. As he grabs their legs, they kick and beat him, not stopping until Courtley dies, at which they flee. While the three return to their respective homes and return to their lives, Courtley's body, left in the abandoned church, transforms into Dracula, who vows that those who have destroyed his servant will be destroyed.
Dracula begins his revenge with Hargood, who has begun to drink heavily and also treats his daughter Alice harshly, furious that she continues to see Paul, Paxton's son. Dracula takes control of Alice's mind via hypnosis and as her drunken father chases after her, she picks up a shovel and kills him. The next day, Hargood is found dead and Alice is missing. The police inspector in charge of the case refuses to investigate Alice's disappearance, citing a lack of time and resources.
At her father's funeral, Alice hides behind bushes and attracts the attention of Paul's sister Lucy, telling her to meet her that night. They enter the abandoned church where Alice introduces her to a dark figure. Lucy assumes him to be Alice's lover but she is greeted by Dracula, who turns her into a vampire.
With Hargood dead and Alice and Lucy missing, Paxton fears that Courtley is exacting revenge and, together with Secker, visits the abandoned church to check for Courtley's corpse. The body is missing but they discover Lucy asleep in a coffin with marks on her throat. Secker realizes she is a vampire and tries to stake her, but Paxton shoots him in the arm, forcing him to flee. While Secker stumbles his way home, Paxton weeps over his daughter's body. When he finally develops the courage to stake Lucy himself, she awakens, and Dracula appears. Alice pins Paxton down and Lucy drives a wooden stake through his chest.
That night, Secker's son Jeremy sees Lucy, his lover, at his window and comes down to see her. She sinks her fangs into his throat, enslaving him while Dracula watches. The vampire Jeremy then stabs his father on Lucy's orders. On the way back to the church, Lucy begs for Dracula's approval but instead he drains her dry and leaves her destroyed. Back at the church, he prepares to bite Alice but a cock crows and he returns to his coffin.
Secker's body causes Jeremy's arrest. The police inspector assumes that he hated his father and stabbed him in a rage. Paul disagrees but the inspector refuses to listen. He hands Paul a letter - "the ramblings of a lunatic" he calls it - in which Secker instructs Paul on how to fight the vampires.
Following Secker's instructions, Paul makes his way to the abandoned church. He finds Lucy's exsanguinated body en route, floating in a lake. At the church he bars the door with a large cross and clears the altar of Black Mass instruments, replacing them with the proper materials. He calls for Alice, who appears together with Dracula. Paul confronts Dracula with a cross but Alice, still entranced, disarms him. She seeks Dracula's approval but he dismisses her. He tries to leave but is prevented by the cross barring the door. His retreat is also barred by a cross which an angry and disappointed Alice threw to the floor. Dracula climbs the balcony and throws objects at Paul and Alice, before backing into a stained glass window depicting a cross. He breaks the glass but suddenly sees the changed surroundings and hears the Lord's Prayer recited in Latin. Dazzled and overwhelmed by the power of the newly re-sanctified church, Dracula falls to the altar, and dissolves back into bloody dust. With the vampire destroyed, Paul and Alice leave.

Question: What does Secker realize about Lucy?
Answer: She is a vampire