Can a cousin write a recommendation letter for graduate school? I have a cousin who used to be a research engineer at MIT until recently, and now is the global manager of a company. While at MIT, he suggested that I should apply for graduate school there (I'm an undergrad in Mechanical Engineering in Brazil). He knows me pretty well and is encouraging me to try. I have a good GPA, 3.6/4 as of now, but could be 3.8/4 by the time I graduate. I also have published papers, some of which will be published in top journals. 
Since he used to be part of the MIT community, would a recommendation letter from him be an advantage for me? Or, because he's family, the comittee could simply consider it biased? 

PS: He was not from the mechanical engineering department.
This is probably not a good idea.  A strong letter of recommendation will make it clear how the recommender knows the applicant.  If it is apparent that the recommender is a family member, then many readers will simply ignore the recommendation. 

Furthermore, a strong letter of recommendation will describe specific situations in which the applicant did something well.  In this case, your recommender may not know you except through your family connection and probably doesn't know enough about your academic or research background to provide such examples.