The term for a second/proxy in an org is "Voice". A Voice is a plenipotentiary, speaking with the full authority of whoever they represent. It's like a vice president, or a deputy governer, but in an era where it's expected that geographic distances and lack of instant communication will prevent consultation, so they are empowered to act with great independence. The Voice has authority, but ultimately they only have the authority from who they represent- they speak with the voice of the head of their organization. That's why it's ok to appoint one that isn't necessarily in the family: because if they are doing something counter to the High Lord's will, they will be removed. Unlike a vice president or a deputy governer, they have no authority of their own or implicit separate duties of their own, but it is expected for them to be leading up projects without much need for consultation and to show initiative. Because of this relative independence, sometimes people appoint those best suited for the job (Thanks Alarice!) as opposed to whoever is up next to get a thing in the family. You can be awesome without being administrative, and conversely you can be very good at administration but not necessarily in a family by birth.
The general egalitarian bent of Arx in the past centuries has contributed to this, and there are certain examples of non-family Voices being appointed in recent history. A Voice, after all, is simply an extension of the will of the High Lord, and speaks with their 'Voice' so long as they do so to serve that will.