Clearly no such term exists that we’ve found, so allow me to make one up: ‘Lifebrother/sister’

An analogue would be ‘your big brother who teaches you about life and such’. It starts to capture the closeness of the relationship. ONe would expect that they remain close confidants all through life; more than a servant. They serve the family not the person?

‘Utbror’; fake (or maybe real) Swedish: ‘Out brother’. Out in the sense outside of the family.

Something like a godparent also comes kind of close,  the unrelated  “aunts and uncles” that kind of thing.

They could well be family, but not ones in the line of succession, but of valued experience.

--
Bruce Johnson
University of Arizona
College of Pharmacy
Information Technology Group

Institutions do not have opinions, merely customs