NO, Jasper and Rosalie aren't related.Jasper uses the last name of Hale because he and Rosalie look so much alike and people assume them to be twins. The ヴァンパイア tend to go with whatever the humans are willing to believe. Rosalie insists on using Hale as a last name, and therefore, Jasper took it as his, too. Jasper's real last name is Whitlock..
NO,they r not related, but in the bk they pretend to be twins,(u know at school and with other people) so that they can make the whole adaption thing easier,i guess!!!