Technically they came from different families before they were vampires, but when they were adopted by Esme and Carlisle, they kind of became sister and brother.
They aren't they just pretend 2 be in Forks well aparently they look like each other but jasper is good looking and roslie ain't....so they said they where twins.... that's how =)
Jasper and Rosalie Hale are not actually blood related. They tell people, however, that they are, and were adopted by Carlisle They tell people of Forks, WA they are twins. Out of all the Cullen...