The Joy of Cooking

I think the central theme of this poem is that everyone has characteristics that make them different.  The author picked the sister's tongue to represent that she talked too much because that's the part of her body that moves the most.  Her tongue is "best with horseradish" which I interpreted it to mean that she cusses often.  The speaker's brother is represented by a heart.  This is not any old heart though; it's small, firm, dry, and muscular. I interpreted these as the brother is malign, ungenerous, and selfish.  Normally people say that others have hard hearts meaning that they are cruel and mean.  The brother's heart "barely feeds two" which means the heart is not as big as other people's hearts who "serve six."  I think the speaker is trying to tell the reader that these two aspects of his siblings annoy the speaker the most.