A little declension changes everything.
Ihre Mutter Julia ist brasilianische Herkunft.
This means:
Her mother Julia is Brazilian origin. (A female person (Julia) IS an abstract thing (origin)). That obviously doesn't make any sense. You used 2 times the nominative declensions, that means you describe what the subject is (subject complement). https://en.wikipedia.org/wiki/Subject_complement
Ihre Mutter Julia ist brasilianischer Herkunft.
Closest (maybe weird translation)
Her mother is of Brazilian origin.
The "r" (genitive) at the end, changes everything.
In grammar, the genitive case (abbreviated gen)[2] is the grammatical case that marks a word, usually a noun, as modifying another word, also usually a noun—thus indicating an attributive relationship of one noun to the other noun.[3]
https://en.wikipedia.org/wiki/Genitive_case
Edit: I'm a German teacher, and I explain this at least once a day. You teacher probably told you: "Always use "sein" with the nominative case."
This is only the case if you describe the person (subject complement)
Ich bin Deutschlehrer. (NOM + NOM)
Er ist ein guter Mensch. (NOM + NOM)
Be careful with structures like your example or:
Das Buch ist mir. (informal German - the book is mine)
"Mir ist schlecht." vs. "Ich bin schlecht." (I am badly off / I am bad)
Sie sind des Todes. (obsolete - They have to/will die)