Actually, except for the Castellano/Dellacroce/Gallo set up, the underboss is usually the boss´s closest confidant, his alter ego and the one who is handpicked by the boss to serve as his second. The consigliere is elected by all the members of the Family and acts as an advisor to all of them. Because of the consigliere´s popularity with the troops, and the fact that a boss cannot remove him from his position unless the captains demands it, his power can reach enormous proportions.

So to answer this topic´s question, it depends on the consigliere´s ability or skill to maintain the member´s confidence in him. If he´s not able to do that, the underboss, because of his closeness to the boss, will surely outpower him.