Yeah its just the author's style.
One more point is that the President of Science in IGO mentioned that all 4 of the Heavenly Kings came from poor country. Now, I doubt the son of the Boss of bishokukai was starved.
It actually might be that all of the 4 Heavingly Kings might be blood related or it might be as I thought that they all are childre of strong Bishokuya, Bishokukai or Saiseya. It might be an interesting turn of events in the future Arcs, when they'll arrive to the Gourmet World.
Let's see, but I believe that Sani might be related to Midora. And there is also a posibility that Staa is related to Midora. The thing is that Staa never showed that he can actually manipulte his hair the same way Sani and Midora can. So for now I will stick with my prediction, but it might happen to not be true. Hope some tipps will be revealed sooner.)
As I said before, I also believe that Toriko and Ichiryu might be related.
I believe it's because of Midora's beard and scars that most of people don't find him looking like Sani. Also the colour of his hair, but still I would say that their facial appearence and body shape look quite alike.
P.S. Not even talking about their similar ability to manipulate their hair.