Bella has a sister who is one year older than her, while Bella is a 17 year old junior, Maddie is a 18 year old senior who happens to have a 4 year old little boy. Maddie's son was an accidental byproduct of drinking at a party.
What if Bella had a sister? They are the same age but Olivia was adopted. What happens when Edward makes everyone leave? What happens when they return? This is Oliva Alison Swan!
Isabella isn't the only Swan sister. She has a little sister that is 6 months younger than her (Charlie cheated on Renee when he was drunk) however her mother died during childbirth so Renee took her when she left with Bella.