Hi,
Brandenburg is a city and a federal state in germany - around Berlin.
My 2nd great grandmother was also a Brandenburg. "Friedchen Marie Brandenburg" There are many Brandenburg in Mecklenburg-Vorpommern area - a second federal state in germany.
I think, that they came once from the city or the area of Brandenburg and went to Mecklenburg-Vorpommern and got probably the name, because they were from Brandenburg.