Both Sanji and Zoro have traits of Luffy, as many have said before, Sanji has the heart of Luffy, Zoro has the determination. Sanji turns enemies into allies, is drawn to help people in need, has a fantastical dream that people laugh at, and takes up leadership when broken into groups. Zoro has the one-track minded straightforwardness, dream to be the greatest of something, unbroken will, and first to the front of a battle. They are literally the Yin and Yang of Luffy.
There is far more to characters and this story than just 'my dad beats yours dad.'