The rarest ingredient of the curse-breaking ritual that will restore the vampires to their former glory is the Petrova doppelganger. The oldest vampire in the world has been searching for this person - who can only turn up in the Petrova bloodline (obvs) - for millenia. So, naturally, the first thing he will do when the doppelganger he discovers escapes from his grasp is to... wipe out the Petrova family and remove any chance of another doppelganger being born.
ETA: Plus isn't it LOLNOTFUNNYATALL that in the 15th century, women were giving birth on their backs mothers used to coach their daughters to push during labour?
*headdesk*