I've never once heard drosje in Norway, just 'taxi'. I've only heard drosje in Pimsleur's audio course and here. I think it's an old word.
Why drosje not en drosje ?
Is "Onkelen sin..." wrong?
'sin' means that the subject owns the thing. The uncle cannot own himself, so there is another person (she) who owns the uncle. In which case you should use 'hennes'.
Thanks! I read the notes for the Possessives again and it all came back.