One of the ways you can tell the "sensible" characters from the "insensible" ones is their speech patterns. Laura et al declaim in flowery language - lots of "dost thous", etc, while those with no senbility, those whom Laura can only pity, speak plainly and to the point.

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Edward: "...dost thou then imagine that there is no other support for an exalted Mind (such as is my Laura's) than the mean and indelicate employment of Eating and Drinking?"

Augusta: "You are too ridiculous (said Augusta) to argue with." (Letter 7)
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Laura: "Base Miscreant! (cried I) how canst thou thus undauntedly endeavour to sully the spotless reputation of such bright Excellence? Why dost thou not suspect my innocence as soon?"

MacDonald: "Be satisfied Madam (replied he) I do suspect it, and therefore must desire that you will both leave this House in less than half an hour." (Letter 13)
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Augusta: "What! (interrupted Augusta) is my Brother dead, then? Tell us, I intreat you, what is become of him?"

Laura: "Yes, cold and insensible Nymph, (replied I) that luckless Swain your Brother, is no more. (Letter 14)
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I always picture someone from the Jon Lovitz School of Acting! (for you old Saturday Night Live fans) declaiming these showy speeches, complete with lots of large arm movements. What fun JA must have had reading these lines aloud to her family.