Other mistake: When Dr. Seward first goes to see Renfield, he gets bitten on the right side of his neck, but then grabs the left side as if in pain. (00:26:00)
Other mistake: When Jonathan and Mina get married, he gives her wine to drink, which she somehow manages to do without removing the veil from her face. (01:20:25)
Answer: It's unlikely they were falling in love with Harker: they are sadistic, baby-eating monsters who regarded Harker as food and a temporary plaything. As for them sleeping in the open, the local populace dreads and avoids Dracula's castle, so there's hardly any fear of intruders. Van Helsing did enter and kill them, but they reckoned, mistakenly, that he too would be too afraid to do so, especially after their horse-mauling escapades the previous night.
Jukka Nurmi