While discussing Jane and Bingley's relationship in Ch. 5:"Happiness in marriage Is entirely a matter of chance. If the dispositions of the parties are ever so well known to each other or ever so similar beforehand, it does not advance their felicity in the least. They always continue to grow sufficiently unlike afterwards to have their share of vexation; and it is better to know as little as possible of the defects of the person with whom you are to pass your life." Charlotte is already practical enough to accept her "share of vexation" and we know she will with her later decision. However, you're right in asking how well does Elizabeth really know her? She responds, "...You know it is not sound, and you would never act in this way yourself." Elizabeth is judging Charlotte with her own set of independent values and assuming she could not go into a marriage without thoroughly knowing and caring for her partner. Unfortunately for women of that era, Charlotte is being as pragmatic as possible without any romantic delusions.