Why does roseile and jasper have the same last name

rosalie left her last name because she is the one who is most attached to her past and her human life so she left it as a sort of link, connection with her humanity... and jasper doesn't care about his last name as long as he's with alice and since it was convenient for the story that cullens tell when they arrive somewhere he took hale as his last name =)

because they are supposed to be twins and it would be strange for them to have all come from the sma family