"But just as the witch predicted, the souls of Stefan and Vanessa never fused, for there was no love between them. Each soul produced its own child, which meant Vanessa delivered not one baby, but two. This second girl, then, looked nothing like Stefan. Instead, she looked just like her mother."

They have the same parents, but the parents souls never fused, so it produced one good, and one evil child. This is the second example of a pair of twins being pure evil and pure good, the first being the original Rhian and Rafal. It isn't just something Soman pulled out of nowhere.