workplace favortism

I work in a family-owned textile mill which employs about 2,200 people. My question about favoritism.
I am a white woman age 36 and am bilingual in Spanish. I have 6 1/2 years seniority on my job.
A couple of years ago, the plant hired a few hispanic workers and needed an interpretor to train and help with various situations pertaining to these employees. I was designated to the job and recieved excellent reviews by management and human resources. My job duties included orientation, time cards, interpreting during disciplinary meetings, assisting in filling out job applications, translating and typing a job description and policy handbook into Spanish, and assisting the plant nurse with drug test administration. This job was temporary and after the employees were trained, I returned to my previous job.
A few weeks ago, a job opening was placed in the local classifieds for a bilingual personel person at this same plant. The only qualifications mentioned in the ad were that the applicants have a GED or high school diploma, some computer skills, and good itrapersonal skills. I applied for the job and was told that my file would be reviewed along with the other applicants and that the most qualified candidate would be hired for the job.
Today , the job was awarded to a young hispanic woman with 1 year seniority from my same department. This woman has an extremely friendly relationship with several members of management and is very attractive.
Three days prior to her promotion, she stated to two co-workers that she knew she couldn't get the job because she does not possess a GED or high school diploma.
Today these same two co-workers asked her again about the diploma issue and how she managed to get the job. She stated that she doesn't have an American diploma, but does have one from Mexico. I can also tell you that prior to her employment in this plant, the only other job she held was as a waitress at the local Huddle House restuarant.
When I inquired of the plant manager as to why I was passed over in favor of the other, less-qualified person, I was told that I have an attendance disciplinary write-up and that company policy prohibits promotion of employees having disciplinaries of any kind. This would seem legitimate if I had not been selected a year ago for a promotion to Quality Control Inspector with the same type of disciplinary on my record at that time. I was told that it didn't matter about the write-up then. Now it seems it does matter.
Would this be considered favoritism or anything else? Would a lawsuit be advisable?