Ammon appears in this verse as the "chief" among the sons of Mosiah. It is possible that he has this position as the eldest, a fact suggested not only by his determination as the "chief," but also because he is listed first in the list of the names of Mosiah's sons given in Mosiah 27:34 (this same list is given in the same order in Alma 23:1, 25:17, 31:6. There is another mention of the first three in Alma 31:32). As noted in the discussion of Mosiah 27:34, the only counter indication against his being the eldest is the initial proposal of the people that Aaron be the king.
We are not given much information about what it was that Ammon did to "administer," but the way the sentence is constructed it appears that there is a softening of the possible meaning of being chief among them. Ammon may be the leader, but it is he who "administers" to his brethren. Ammon appears to understand the principle of leadership that the Savior taught:
Matt. 23:11: But he that is greatest among you shall be your servant.
Rather than make himself the subject of service, Ammon apparently renders service. This service included blessings, and possibly directions, though we are told only of the blessings. It is probable that as the "chief" among them, Ammon was the one to direct the nature of the separation of the brothers, but the text is careful to emphasize his service rather than his orders.