The pinky promise ( we meet again)

There were two best friends, who love each other, went to the same school, got married at the same time, their wives got pregnant, give birth at the same year but some months apart, a girl and a boy. But after 4 years the girl's mother died. This made one of the friend relocate to England. But unknown to them that the children promise to marry each other

18 years later,a lot of things have change, the boy refuse to get into any relationship. Did I forget to mention, the two friends are the top billionaire in the world.
The girl's father decided to go back to New York.
What will happen when the see each other, will keep to their promise after knowing each other secret
What is the secret between the friends and the source of their wealth