JA tells us that Lizzy doesn't know Lady C and her daughter before:-

Mr. W:- "You know of course that Lady Catherine de Bourgh and Lady Anne Darcy were sisters; consequently that she is aunt to the present Mr. Darcy."

Lizzy:- "No, indeed, I did not. I knew nothing at all of Lady Catherine's connexions. I never heard of her existence till the day before yesterday."

Then she speaks very ill about them:-

lizzy:- "Mr. Collins," said she, "speaks highly both of Lady Catherine and her daughter; but from some particulars that he has related of her ladyship, I suspect his gratitude misleads him, and that in spite of her being his patroness, she is an arrogant, conceited woman."

She refers her ill opinion about them to the particulars that Mr. C mentioned about them. I don't think at that time she is allowed to give her opinion about them since she didn't meet them, also it appears from Mr. C's behaviors that he is not a man whom one builds his own opinion according to something he said. Further more Mr. C always praises the Lady and her good deeds, so from where she got that she is an arrogant, conceited woman.