The introduction to Alma’s counsel to Corianton suggests that perhaps all of the brothers were present at the same time for these blessings and counsels. Shiblon received a shorter version of Alma’s conversion story, and Alma skips the introduction he had used for both Helaman and Shiblon. Both of those sons were basically faithful, but Corianton is more problematic. Therefore, there is more to be said, perhaps because Corianton needs more counsel.
When Alma asks if Corianton has observed the steadiness and faithfulness of his brother, it appears that he is speaking of Shiblon. That will be appropriate because Corianton’s issues began when preaching to the Zoramites. Helaman did not go to the Zoramites, but Shiblon did, and Alma praised his efforts there, in spite of having been driven out when people threw stones at him (see Alma 38:4). Shiblon’s example among the Zoramites will contrast with Corianton’s experience among the same people.
Note that Corianton “[did] go about boasting in [his] strength and [his] wisdom.” Alma had made a point of telling Shiblon that he should not “boast in [his] own wisdom, nor of [his] much strength.” Thus, Alma sets up Shiblon as one who was tempted with the same temptations as was Corianton, but he did not succumb to them.