I have great news about my sister case.
I was waiting the result to post over here.
A couple of weeks ago we found my mother passport with the date of return to Brazil, including my sister's name.
Along with the passport we found a vaccination card from my sister, when she was a little over a year old.
With some additional documents attached, it was enough proof to the my sister's children acquire their mother's nationality.
They just received the "Consular Report of Birth abroad" and the passports at home.
And I want to thank those who helped us on this forum, it was of great help to us.