We all form them and they do have a tendancy to color every experience we have of a person that comes after them.

Now it really remains to be seen whether Elizabeth is wrong about her first impressions:

Bingley's sisters do not really make any effort to be nice to anyone unless they have a paritcular reason for doing - CH 9 has some good examples of this - "Nothing but concern for Elizabeth could enable Bingley to keep his countenance. His sister was less delicate, and directed her eye towards Mr. Darcy with a very expressive smile."

Here Bingley is making an effort to make his house guest comfortable and not be rude to Mrs Bennet - but through out the Bennet's visit Miss Bingley is barely civil.

Also in Ch 6, the SS make an effort to become friends with both Jane and Elizabeth - prior to Mr. Darcy showing any interest in E. As soon as Darcy expresses an interest in E., Miss Bingley begins openly disliking Elizabeth.

In regards to the SS, Elizabeth's judgement seems spot on.