The Cullen family, for outside appearances, are the foster sons and daughters of Dr. and Mrs. Cullen. Jasper's surname as a human was originally Whitlock, but upon joing the Cullens and arriving at Forks, Jasper used Rosalie's surname, 'Hale' and they are known as 'biological' brother and sister, likely because of their similar physical appearance as blonds. Emmett, Edward, and Alice are 'biological' siblings with the Cullen name, again, because they are dark-haired and more likely to be accepted by the outside humans as being related. In actuality, none of the Cullens are related biologically. The 'foster siblings' are actually couples: Rosalie Hale and Emmett Cullen, and Alice Cullen and Jasper Hale, and for appearance's sake to avoid arousing suspicion from humans, they could not all be related.