I seem to recall that the characters implied, or maybe they even said that Millicent wasn't a first cousin. I know that it wasn't frowned upon to marry first cousins even in the Victorian age, and this was even further back than that. I'm not sure when people figured out that you probably shouldn't marryyour first cousin, or when the law started to prohibit the practice.
A vampire biting is not a sexual act.
No wonder so many of them look alike!