Are the Three Sisters in the au too? I assume Van Helsing and Mina were not able to go there and kill them.
They are! Their names are Elizabeta, Rosalind, and Beatriz in this AU. I’ve drawn them in this picture, with Beatriz to the left, Elizabeta in the middle, and Rosalind to the right. In this AU, they're more sympathetic to the Crew of Light but not enough to help them escape. There are some lines you just don't cross when it comes to Dracula.
Elizabeta is the oldest and the first vampire he successfully turned, and she is the only one to have been somewhat willingly turned. She was originally in an abusive marriage with few family or friends and unfortunately found companionship in a certain mysterious boyar. After her husband stabbed and pushed her down a flight of stairs, she was found by Dracula, who promised that she would live if she drank his blood. She's the most caring and patient toward the new vampires, as she has gone through this process multiple times before.
Rosalind was a peasant who fell into Dracula’s clutches when she was trying to escape charges of witchcraft and he offered her protection, but when she was in his castle and realized what he was going to do to her, she tried to fight back only to lose in the end as she didn’t know how to kill a vampire and was promptly turned by Dracula. She is most decidedly not Van Helsing's biggest fan at first, as he was the one who nearly killed Elizabeta and Beatriz as well as herself, but does want to protect the rest of them nonetheless.
Beatriz was a young noblewoman who was trying to avoid an arranged marriage to someone she despised. Dracula befriended her and she jumped at his offer to take her far, far away where her family wouldn’t be able to track her down. When she realized what he was and the danger she was in, she tried to escape and hid from him only to be drained and turned. She's thrilled to have new company in the castle, even though she knows they'd much rather be anywhere else.
As for Van Helsing and Mina, in this AU, the primary reason things go differently is because of a snowstorm, which leads to a delay where night falls before they can properly kill Dracula. Van Helsing also needed to carry Mina in the snow, without horses, to the castle in order to not leave her alone and unconscious in the storm, which also burned a lot of time on their end as well and caused it to be late enough that the vampire ladies were able to wake before he could kill them.