Told by family, his siblings
If he had siblings - when did they tell you he had gone to NZ as they are all presumably deceased by now?
There are a lot of outward passenger list entries for the name James Grant with a birthyear 1909/1910 - why did you select the 1929 entry?
He would only have been 18 at that time so presumably any marriage would have been in NZ?
Surely his siblings would have known the names of his wife and children? What about his siblings children - he was their uncle so do they not have any details about his family?