not a teacher
I think "gingers" are people with red hair, freckles, and pale skin who have some problems with their survival. And for a stepchild, who is a child of an early marriage, is also very difficult to survive having a stepmother or a stepfather. Hence a ginger stepchild might be taken as a symbol of hard surviving. The ginger stepchild might be a concrete red-haired stepchild with a lot of problems.
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