All of the Cullens are related to Edward, but not genetically. Carlise and Esme are his "adoptive parents" and Jasper, Emmet, Rosalie, and Alice are his "adoptive siblings".
Only by marrige...and some vampire venom xD Bella married Edward so he last name became Cullen. The vemon part is when Edward changed her because when she (a human) was pregnant with a vampire baby,...