They were 'adopted' so they are not actually related by having the same biological parent(s).
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Technically, Edward is the oldest. Carlisle turned him before Emmet and Jasper were vampires.
not biologically. Rosalie's last name is Hale; Jasper's last name is Whitlock. Since both Jasper and Rosalie both have blonde hair, it was easy to pass them off as faternal twins. Rosalie is stuborn and hard-headed so they chose to have Hale as the last name instead of Whitlock (Jasper's). Since Alice is a Cullen, and Jasper is married to Alice, Jasper couldn't become a Cullen.
In terms of when they joined the family, Edward is oldest and Jasper/Alice are the youngest. In terms of when they were changed, Jasper would be the oldest, but because we don't know exactly when Rosalie and Emmet were changed we can't tell who was the youngest, if it was Rosalie, Emmet, or Alice.
In twilight Edward has two sisters: Alice and Rosalie. In the before they found Alice or Rosalie, Edward had to pretend to be Esme's brother. How could you not know this it is in the books!!!!!
Same as now well at least for Rosalie Hale and Jasper Hale. Edward, Alice, and Emmett, changed their last name to Cullen either because they don't remember or just since they were adopted want the name of Carlisle and Esme Cullen haha I <3 TWILIGHT SOOOOOOOOOOO MUCH!!!!!!!!!!