TL;DR: Upper-class family has no sons, so after extensive vetting they adopt somebody -- almost always a daughter's husband, aka son-in-law -- to become the son

TL;DR: Upper-class family has no sons, so after extensive vetting they adopt somebody -- almost always a daughter's husband, aka son-in-law -- to become the son and continue the bloodline/business. This is easily accomplished since the Japanese koseki family registry is primary-keyed by last name. Usually the wife takes her new husband's name and joins his koseki, but if they do the reverse, then the son is now part of the wife's family. https://en.wikipedia.org/wiki/Japanese_adult_adoption https://en.wikipedia.org/wiki/Japanese_adult_adoption https://en.wikipedia.org/wiki/Koseki https://en.wikipedia.org/wiki/Koseki