"Jouw nicht wordt morgen twaalf."
Translation:Your niece is turning twelve tomorrow.
I agree! People turn red or turn nasty or take a turn for the worse - it's become a mildly pejorative term, in my vocabulary, at least. But "becoming" twelve is a bit of a problem as well. I might simply say "Your niece is (or will be) 12 tomorrow" and leave it at that.