One possible plot twist could be that the twins are actually not related by blood but are part of some mysterious experiment. This would turn their whole identity and purpose as detectives on its head.
A really interesting plot twist could be that the villains they are after are actually working for a higher power that the twins have a connection to, perhaps a long - lost relative or an organization that their family was once a part of. This would create a complex web of relationships and motives that the twins have to untangle.
The villains are also important main characters in a way. They could be other spirit detectives who have gone rogue or some sort of dark entities in human form. Their actions and motives drive the plot forward as the twins try to stop them.
One interesting plot twist could be that the forgotten twin has a different magical ability than Harry. For example, while Harry is good at Defense Against the Dark Arts, the twin might be a natural at Potions or Charms. This could lead to a situation where the twin is initially seen as an outcast in Hogwarts because their skills don't match the typical Gryffindor traits. Another plot twist could be that the forgotten twin was actually hidden away by a powerful dark wizard who planned to use the twin's power for their own evil purposes. This would put the twin in a difficult position when they discover their true past and have to decide whether to side with the forces of good or be influenced by the dark wizard's plans.
One interesting plot twist could be that Hermione and Harry discover their relation much later in their Hogwarts years. Maybe they've always had a special bond but just attributed it to their friendship. Another could be that their shared heritage gives them unique magical abilities that no one else has, like a combined spell -casting power that can defeat the darkest of dark wizards.
One interesting plot twist could be that the twin sisters are the key to a prophecy that involves Edward. They have special powers that are only revealed as the story progresses. This makes them a target for both good and bad forces in the fanfiction world, and Edward has to protect them at all costs.
One interesting plot twist could be that they discover their twin bond gives them shared magical abilities that are unique. For example, they might be able to combine their powers to create a new kind of spell.
Well, in many family business novels, a common plot twist could be the discovery of a long - lost relative who has a claim to the business. This can throw the whole family dynamic and the business operations into chaos.
One interesting plot twist could be when the main character discovers that the treasure they've been hunting for all along is not gold or jewels, but a map leading to an even greater mystery. This completely changes the direction of their hunt.
One interesting plot twist could be that the vampire hunters turn out to be vampires themselves in disguise. This completely flips the expected dynamic and adds a whole new layer of mystery and danger.
One interesting plot twist could be that their different upbringings within the wizarding world lead to a complete role reversal. Harry, raised in the Malfoy family, might be more prejudiced at first, while Draco, having grown up with the Weasleys perhaps, could be more open - minded. This would shatter the initial expectations of their characters and lead to a complex exploration of nature versus nurture in the wizarding world.
Perhaps a character who is initially presented as an enemy turns out to be an ally. In 'blood secrets fictional novel', this could happen when the character's true motives are revealed. Maybe they were only acting as an enemy to protect someone or something important.