Katrina and Spellman twins
Every member of the Spellman family having a twin means the family is twice a big as you might think it is. How is that kept a secret from Spellman children, particularly those who are raised knowing that they and their families are witches and are exposed to more witch and Other Realm stuff? Being born with a twin means every mother who gives birth to a Spellman baby gives and gives birth to twins. Teenagers are sure to notice that. Twins are presumably separated early to preserve the secret. Katrina raised in South Dakota. How was it decided which twin to keep with their parents? Who the heck raised Katrina? Suppose the evil twin is raised with his/her parents and the good twin never knows them until their basically an adult. Supposed Hilda or Zelda was raised with the other's evil twin. They wouldn't be able to know each other until they're old enough. Seems like all the good twins were coincidentally raised by their birth parents and among the bulk of their extended family.
(this signature was absent on picture day)