ip man's brother real or fiction ?

I wonder the same thing.

As well trained Japanese people tend to be are, it's still unrealistic for children to be trained as spies in that early age.
Their minds are still young and DEVELOPING.

If you smuggle a child as a spy to live with a chinese family, he will eventually over a time start to get used to living as a chinese and adopting his life around it. His mind will start to grow and adapt into his new surrounding (ie living as a child in a chinese family) and after a while, he will forget about his past life.

Given the many number of years Tin Chi lived with his adopted family (from child to adult). It would be too difficult for a child to keep remembering his orders as a japanese spy.
Unless of course, the child has a means of regularly sneaking out in secret and keeping constant contact with his army over all these years (to keep reinforcing and "reminding" him of his mission).
He would have forgotten that he's a spy after so many years of growing up.