Nobuyuki and Katsuhito (Yosho) look similar because they are actually distantly related. Yosho has had several wives since moving to earth 700 years ago. His children have had children, who have also had children, and Nobuyuki is one of those distant offspring. Intermarriage of distant relations in royalty is quite common (Ayeka was originally engaged to marry Yosho, her half brother [OVA Ep. 2]), thus Achika's marriage to Nobuyuki would not seem unusual to the Jurai.