Both the relationships termed mother and aunt in English fall into the same taxon "Mother-Aunt". This does not mean that the users of this taxonomy are confused about the concept "Birth-Mother," only that it is a subcategory.
Both relationships identified as mother and aunt in English are part of the same taxon "Mother-Aunt." This does not suggest users are confused about the "Birth-Mother" concept but rather that "Birth-Mother" is a subcategory within this taxonomy.