Alma, though addressing Corianton as “my son,” does not speak his name nor begin with the Nephite foundational promise. Prospering through righteousness is, evidently, a less certain possibility for Corianton than for Helaman and Shiblon.
Alma had alluded to Helaman’s blessing when addressing Shiblon (Alma 38:1), and here refers to Shiblon (and by implication to his blessing). Alma is reminding Corianton of his brother because he hopes that their biological bond will lead to a strong spiritual bond. Shiblon, rather than Helaman, is held up as an example to Corianton since Shiblon’s achievements and responsibilities more closely match Corianton’s than the perhaps-impossible status of the firstborn. In ancient patriarchal societies, the firstborn’s rights are paramount, and it is the violation of those rights through Yahweh’s will that makes so unusual the stories of Joseph in the Old Testament and Nephi in the Book of Mormon.
Shiblon, however, was not privileged by birth, but only by personal merit. As such, he is a perfect example for Corianton, who needs to learn to obey for righteousness’s sake, and not for position or responsibility.