I think that war brides had no particular country of origin other than
outside the U.S. The missing part of the story is where was mom when
she met stepdad? If she was already in this country then the couple
engaged in the time honored and common practice of working the U.S.
immigration system to their benefit. If they met in Mexico and he
brought her home ITSM she is no different than a British or French or
Italian "war bride."