With the existing immigration
laws, you would be eligible to file a I-130 petition for your fiancee that would allow her to obtain a green card
but unfortunately you would not be able to petition for her daughters because they both would be over the age of 18 at the time of your marriage. In order for a US citizen to file a petition for a step-child, the marriage creating the step-child/step-parent relationship must be created prior to the child's eighteenth birthday.
However, your wife would be eligible to file for her daughters but they would not be eligible to receive an immediate green card in the United States and would need to receive a green card abroad which currently takes at least 2.5 years in most cases for a lawful permanent resident filing for a child under the age of twenty-one. Once a child is over twenty-one, he or she is no longer considered a child for immigration purposes so the twenty year old will probably not be eligible by the time the immigrant visa
The process to file a petition for your fiancee is outlined briefly below:
You would file the I-130/I-485
Packet (Petition for Alien Relative) and your future wife could adjust status in the United States without returning to Brazil. The entire process takes a little over a year and would require that your future wife take a Biometrics exam and that you and your wife be interviewed by a USCIS officer who will determine if she married for the purpose of obtaining permanent resident status. If approved, she would be issued a conditional permanent residence card because she has been married less than two years at the time that you filed the petition.
The I-130/1-485 packet is found at the USCIS website at uscis.gov.
You and your future wife should file the following forms with supporting documents:
I-485-green card application
I-765 -Work permit application
G-325A-biographic form for both of you
I-864-Affidavit of support