Why can't friends or family members write recommendation letters for graduate admissions?
Because they cannot be objective.
Because they do not have firsthand knowledge of your academic abilities. 
Because they do not have the background necessary to compare you to other graduate school applicants.


Also see Kisses of Death in the Graduate School Application Process, page 2, "Harmful letters of recommendation," subsection "Inappropriate sources," and Protocol for writing a recommendation letter for someone you only know on a personal basis.