From the Brazilian Embassy website:
QUOTE
Requirements for Permanent Visa
Eligibility:
Family reunion;
Transfer of residence following retirement;
Personal investment in Brazil;
Intra-company transferees to work as managers, directors, or executives;
Job offer at a Brazilian research, scientific, or academic organization based on field of expertise;
Manager or director of a religious or social assistance organization.
Procedures for Family Reunion
Submission to the Consular Service of letter informing the reason for moving to Brazil, together with the the following documentation (all supporting documentation that is not originated in Brazil must be authenticated by the Consular Service. Copies of Brazilian supporting documents must be notarized by a "Cartório" in Brazil. Original Brazilian documents are not required notarization):
from the applicant:
Two visa application forms per applicant, properly filled out (front and back) and duly signed (parents must sign for those under 18 years old);
two recent 2" x 2" passport-type photo, in color or black and white, front view, full face, light background (snapshots are not accepted);
authenticated copy of the passport pages of identification;
as applicable, marriage certificate or birth certificate;
recent (i.e. less than 90 days old) non-criminal record issued by FBI - Federal Bureau of Investigation;
proof of residence for at least one year in the consular jurisdiction;
from the sponsor:
copy of identification card, CIC (or a signed and legalized statement, informing the reason for not filing Income Tax in Brazil) and voter title (for Brazilians onlyl);
formal affidavit ("compromisso de manutenção ") drawn or authenticated at a "Cartório" or at a Consular Office on the applicant's behalf;
from the applicant or sponsor:
document attesting to a confirmed job offer in Brazil, or proof of financial capability, or formal affidavit guaranteeing that applicant will be financially supported by an immediate family member, resident in Brasil.
Tecnically your in-laws are immediatly family members.