1- What is the difference between Shikai and Bankai?
Shikai is first stage of zanpakuto, while bankai is it's second (final) stage.
2- Why are there 2 Kenpachi in this manga?(Unohara and Zaraki) It seems Zaraki had that name before even before knowing Unohara. Was it random?
Kenpachi is title given to strongest shinigami in generation (by some translations) or title given to shinigami that killed most
characters (by other translations).
Unohana was first Kenpachi. Kenpachi is also captain of 11th division and Unohana was first captain of said division. She left division and became captain of 4th division, after she learned healing kido. Tradition is that shinigami fight with captain of 11th division, and if he kill him he can sucess them on that position. Since Unohana left position, second Kenpachi got position of captain without fighting. Third Kenpachi killed Second and success him, Fourth killed Third and so on. Zaraki Kenpachi is 11th Kenpachi and he killed 10th in order to became captain. We can assume that all Kenpachis were litterally fake, since 2nd kenpachi never killed 1st. Zaraki Kenpachi finaly in the war killed 1st Kenpachi, and earn his title fair.
Kenpachi Zaraki wes nameless soul, with pretty high potential. He was named Kenpachi Zaraki because, he killed a thousand of souls (Kenpachi) and because he was born in 80 district called Zaraki.
3- Are Rukia and Byakuya real brother and sister?
No, Byakuya adopted Rukia. Rukia was Byakuya's wife (Hisana) younger sister.
4- Who are the 5 war potentials?
5 War Potentials are 5 characters that are threat to Yhwach (somehow). Out of 5, three were revealed and they are Aizen, Ichigo and Kenpachi.
The Soul King is the king of Soul Society, whose existence in Soul Society is as symbolic as it is absolute. Urahara details that without the Soul King's existence, Soul Society would be rent asunder. The Soul King is the "linchpin" and if that linchpin is lost, then the world would simply crumble away and that is the way of the world.