It is quite simple. I have listed some sibling sets (some are real, some are made up), and you name the next baby (a name for a boy and a name for a girl).
Jacob,
Sophie,
Karen,
Karoline,
Harald,
Molly,
Maia,
Lucy,
Emma,
Tara,
Henriette,
Christina,
Marianne,
Nathaniel,
Katharina,
Reuben,
Sarah,
Anna,
Rasmus,
Catherine,
Sarah,
Rachel,