Literary: "Now in this thing we do rejoice," refers to the words that Jacob is able to write that will be preserved for his posterity and for the Lamanites. There are elements of parallel and chiasm in this two verse set, but if there was a formal chiasm in the original, it is not well preserved in the English.
The elements of the posterity and the Lamanites are given in reverse order and the theme of joy appears as the pivotal meaning, but there is a repeated element concerning the fathers of the posterity that is directly parallel rather than inversely parallel. It is possible that this is an artifact of translation, but it is equally likely that is simply is not a crafted chiasm, and uses the reversal of elements simply because they lend some interest to an otherwise directly parallel set. The directly paralleled sets typically have a reinforcing function, and that is also lacking in this set of lines. As lines that appear to be part of the aside, it is easy to see these lines as the result of a direct entry onto the plates rather than a constructed sermon. We would expect more formal structures in more planned setting. These parallels and reversals suggest that Jacob was very comfortable with this type of structural presentation of ideas, with the general structures appearing even when the formal literary structure does not.