This is where that Severus Snape had two daughters that are identical twins and he couldn't tell them apart. When Nina Snape died after her two twin daughters died, Severus kept his twin daughters secret until it was too late. When the Hogwarts School of Witchcraft and Wizardry have found out that Severus Snape had two daughters: Kaitlyn and Diana Snape. Kaitlyn and Diana are identical and Severus can't tell them apart. When Kaitlyn and Diana were 2 years old, Severus had a hard time which shirt goes to which.
When Voldemort found out that Snape had two twin daughters that are identical, he wanted both of them. But Severus wouldn't allow it. But Voldemort wouldn't take no for an answer. So he sent dreams to Kaitlyn Snape and telling her that he wants her to become to be his side and be his followers. Then Kaitlyn's twin, Diana woke up Kaitlyn plenty of times, and telling her not to go to the Dark Lord.
After every single night, Kaitlyn is tired of her twin waking her up. Severus wouldn't let his daughter go, until she was gone missing. The whole castle got up and looked everywhere for Kaitlyn. After four hours of missing, Kaitlyn came back from outside. Severus took his daughter Kaitlyn to his room and looked at his daughter's eyes.
And then looked at his daughter's arm, he removed the clothing from his daughter's arm and found the Dark Mark on his daughter's right arm.
And that is when it all started.