I just finished a full campaign as the Julii, and I had a diplomat going around bribing Brutii armies to get them out of the way, and every one of them joined my faction, rather than disbanding. None of them was led by a family member, they varied from 1 unit to about a dozen units, and my diplomat wasn't anything special skill-wise. But when bribing enemy armies earlier in the game (from non-Roman factions), I'd found that they would disband unless led by a family member.