I read a nice theory about Kuma being Kaido's brother that I wanna share in this forum. (All the credit goes to the owner.)
Now to the theory ->
Kaido & Kuma are brothers
#1: The word Kaido can mean "church", "chapel", "synagogue", "tabernacle" in Japanese.
Kuma is a man of religion. He carries around his bible and he has a bigass cross on his jacket.
#2: There is a cross in the old lady's drawer
This island belongs to Kaido. It makes sense for its residents, who are under the care and protection of Kaido, to also be religious.
#3: Assuming that the man on the cover of chapter 693 was Kaido, he resembles Kuma to an extent.
He's like a hairier, wilder version of Kuma. Also the dark, long hair looks similar.
#4: Cyborg theme
Kuma is a cyborg and many fans believe that Kaido is also a cyborg. The island where X-Drake attacks Scotsh (a cyborg!) has a cyborg theme as well.
Last thought: Perhaps Kuma did something to Kaido that made him, or forced him, to become a cyborg to survive. Then Kuma decided to become a cyborg to punish himself?
What do you think about this? Let's discuss.