we have a nine week old st poodle and a two year old both bitches why does the oldler one nibble the little ones neck

I am no expert but i have been told that this is a sign of affection and that it is the older bitches way of mothering the youngster as that is what mums do. But i am sure someone with more knowledge will be along soon to put me right!!!!

yes, I was going to say the same. A sign of affection ( as long as they are not savaging each other :) )
I know a lot of dogs who nibble each other, I wouldnt be worried about it.