In this member's situation, she can list her mother's three biological families, add her mother's husband's prior children as her mother's step children. and that her mother's prior children are her husband's adopted children, etc.
Add Relationship Type in Family Search | LDS Family History Tutorials
https://www.youtube.com/watch?v=9vrJNrg8YvQ
The easy way to add the children to another set of parents is to go to the Person page of the shared parent and copy and paste the IDs of the children from one spouse's family into the other spouse's family.
Note that prior to the 1900s, adoptions were often informal, without any legal paperwork. But the relationships were still adoptions.
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