I did not find her, or the rest of her family, on a cursory look at
http://www.freecen.org.uk/cgi/search.pl - you might like to make a more thorough search there in case I missed them. Or at
www.scotlandspeople.gov.uk.
If, however, the family is not in the 1841 census, this suggests that they may have left Scotland before that. On the date of the 1841 census (7 June 1841) she was still only 18 (if alive) and I think it's unlikely that a family would leave an 18-year-old unmarried daughter behind when travelling to NZ.
She could, I suppose, have married unusually young, and stayed behind with her husband.
According to the MI book, there are no Watts commemorated on stones in Barry.
You could look for a death at
www.scotlandspeople. There are few records of deaths before 1855, but if she did stay in Scotland, and survived until she was 33, there should be a death certificate, which should be indexed under her maiden name.
FWIW I think it is more likely that she died before the rest of her family emigrated.