Fidel and Raul obviously have many physical differences. A number of authors - including Brian Latell, Norberto Fuentes and Ann Louise Bardach - have written that Fidel and Raul may have different fathers.
Fidel's father was Angel Alejandro Castro Ruz. That is clear. The authors say they believe Raul's father may have been Felipe Miraval, a chief of the Rural Guard where the Castro family lived. Miraval, nicknamed "El Chino," or The Chinaman, did not have a lot of facial hair. Nor was he particularly tall, a trait he could have passed along to his son.
I don't know if that theory is true or not, but Fidel and Raul Castro do look different, as shown in this photo, which hangs in Havana's Hotel Nacional.