The title of Chieftain is earned and kept through triumph in single combat, not by birthright.
R S AP Special Rules
Chieftain’s Blade Combat S+1 -1 Armour Bane (1) Magical Attacks
Notes: The Chieftain’s Blade may only be taken by a model in a Wolves of the Sea Army of Infamy. Whilst engaged in a challenge, the wielder has a +1 modifier to their rolls To Hit.