Is what your sister claimed sort-of correct anyway? An English middle name is generally a given name that can perfectly well be totally unrelated to family history.
It's my understanding that Brazilian (and Portuguese) "middle names" are usually if not always family names, are they not?
Insisting that they are part of your surname sounds like it might be the easiest way to get it accepted - it is common enough in Europe to have surnames with spaces. E.g. a Dutch surname like "Van der Vaart", or "Van Helsing", so it's quite likely various agencies etc. run into that often enough.
It's quite usual to have two family names, one from your mother's side and another from your father's. Usually the father's family name comes last, but it's by no means a rule, more of a social norm (and somewhat fading).
Additionally, it's also perfectly reasonable to have two or more "first names".
The consequence of all of this, and having four names myself, is that I'm always confused what I'm supposed to fill in as my first and last names. I end up using just my very first and very last, but i always feel like I'm being disrespectful towards my mother's family for the omission.
Something similar used to be common with the english too. My great-great-great-great grandfather was "Thomas Tindall Gore", and Tindall was his mother's maiden name. This sort of thing was quite common all throughout the 1700's and 1800's, but seems to have went out of style in the last hundred years or so.
I actually assumed, well into my teenage years, that people's middle names were their mother's maiden name, as that's the case for me, my sister, and my cousins.
Another interesting one I would see a lot is <mother's father's name> <father's father's name> <last name>.
For example, My dad was Charles Marvin Gore.
His mother's father was Charles Maltbie.
His father's father was Marvin Gore.
I don't think I ever saw the two modes combined though, so no example of a Charles Marvin Maltbie Gore. Although that is just in my family. It might have happened in others.
As the sibling comment says, yes she is sort-of correct. Our names are in the format:
<First name> <Middle name> <Grandmothers first name> <Grandfathers first name> <Surname>
So its kinda like a family name, but its not like everyone in our family will have the same family names. My cousins don't share any names with me as they are named after different people.
It causes trouble insisting they are all part of your surname. She recently had issues because the officials dealing with her medical exams assumed only the last name was a surname. So she had call and find someone with the necessary authority and present her birth certificate to get them to cancel the previous transcripts and issue new ones under the correct name.
It's my understanding that Brazilian (and Portuguese) "middle names" are usually if not always family names, are they not?
Insisting that they are part of your surname sounds like it might be the easiest way to get it accepted - it is common enough in Europe to have surnames with spaces. E.g. a Dutch surname like "Van der Vaart", or "Van Helsing", so it's quite likely various agencies etc. run into that often enough.