Stepchild - Kakeisi

…In the end, these were just ways to advance the marriage negotiations in an advantageous manner. [Ueno Shuji]
[China]
In old China, when there was no biological child, a person of the same family (and therefore necessarily with the same surname) of the same generation as the child that should have been born (called the same sect's Zhaoboku equivalent) was selected as the successor for ancestral worship and for inheriting family property, and this person was called an heir (also a stepchild, a supersuccessful stepchild, or a supersuccessful stepchild). It was common for the person to designate someone close in degree of kinship while alive, but sometimes a widow would make the selection after the husband's death, and if a couple died or the wife remarried after the husband's death, a member of the same family could appoint an heir, calling it an end of the line.

*Some of the terminology explanations that mention "superchild" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…これらも結局は縁談を有利に進める手段であった。【上野 秀治】
［中国］
　旧中国では実子のない場合，同族のうちから(したがって必ず同姓である)生まれるべきであった子と同じ世代にあたるもの(同宗昭穆相当（どうそうしようぼくそうとう）という)を選んで，祖先祭祀また家産相続のための承継人とし，これを嗣子(また継子，過継子，過房子)と称した。被承継人が生前に親等の近いものを指定するのが普通であるが，夫の死後に寡婦が選定することがあり，夫妻が死亡，また夫の死後に妻が改嫁した場合には，同族の人が継絶と称して嗣子を立てることも可能であった。…

※「過継子」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報