I came across this issue while describing in writing the relatives of a woman commonly known in the West as Princess Haya.
Princess Haya is the youngest of four current wives of the current ruler of Dubai (and the sixth person to ever be once of his wives as he divorced two of his previous wives). Princess Haya has two children, both from a union with her current and only husband (who is the current ruler of Dubai). Her husband also has had 28 other children, collectively, with his five previous wives (three of whom he is still married to and two of whom are still living but divorced from him), each of whom has at least some children from their marriage to the current ruler of Dubai.
Princess Haya is also a child of the third wife of the former ruler of Jordan. Her mother died when she was three and her father died in 1999, but so far as I know, while she was raised by her father's family, she was not legally adopted by another mother. Princess Haya is the half-sibling of the current King of Jordan and many other children of her late father. So, this is clearly a pervasive issue for her extended family. Having received almost all of her education from elementary school age to attending college at Oxford University, in England, she surely has described it to people in English many times and there must be some consensus term to use for her various kinship relationships.
Princess Haya's relationship to the current King of Jordan is called in every account that I have seen, "half-brother" (rather than "stepbrother"), and likewise the relationship of her two children to the twenty-eight other children of their father by other wives is always called "half-sibling" in every account that I have seen in English, rather than "step-siblings" (I don't speak Arabic which is her first language and the first language of her husband, so I don't know what term is used for that relationship in Arabic).
But, what is the correct term for the relationship between Princess Haya and her children's twenty-eight half-siblings who are children of her sister wives?
My first instinct would be to call them "stepchildren", but in the monogamous world, stepchildren are usually children of your current spouse who are not related by blood to your own children and your children, and the children of your current spouse from a previous relationship and your children from a previous relationship is called "step-siblings" rather than as "half-siblings."
Her children's half-siblings certainly aren't properly described as her children, however. And they are children, who are not her children, who are children of her current husband with a different wife, which is also true of stepchildren.
An almost identical issue can arise in the monogamous world, for example, if a man has a child with his first wife who dies and then marries and has a child with his second wife, and indeed that relationship historically used to be quite common.
But, the only good literary precedent I can think of that comes close is Cinderella, and the children of Cinderella's stepmother were not children of her father, so it provides little guidance. I also can't think of a term other than "stepchild" to use for this relationship
So, to recap, what is the proper term for the relationship between a wife and the children of her husband from another wife, who are related to the wife's own children with her husband as half-siblings?