Kaido wanted to be JoyBoy, he failed.
He heard a prophecy that certain woman was going to give birth to the next JoyBoy.
He got into a relationship with that woman but had a girl. Thats why he hates Yamato and calls her his son.
That woman 10 years later had another child.
Yamato is actually Luffy's sister.
Sounds exactly like something Oda could pull off his ass.