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.
In English, both terms for mother and aunt fall under the same taxonomic category called "Mother-Aunt." This doesn't mean users of this classification system are unsure about the "Birth-Mother" concept; it's merely that "Birth-Mother" is a subcategory within this broader grouping.