Help with a legal question about my part time job?

Ok here is the problem. I am a full time college student who is working part time at a fast food store to help pay some of the bills for school and otherwise. I started a year ago, and everything seemed to be fine. I had enough hours, managers who respected me and backed me up on my work performance. But then around late September they hired this new girl. She has been around our town, she has almost worked at every fast food store in our town, and can't seem to stick to one place. Ever since she has been hired, my job there has been absolute hell. I have been cut back to 9 hours a week(barely able to make a living off that!) and I found out why from one of my coworkers. She has been talking about my mother-in-law in a negative way. My mom in law is a manager at a store that requires a drug test. She went and applied at the store, and failed the drug test. She knows my mom in laws last name, and apparently its my fault she never got hired. So she has talked to the manager that writes the schedule, and claimed stuff about me that never has happened and as has asked her to give her the hours simply because she can shoo customers out of the store 5 minutes prior to closing( our owner does not condone us "shooing"), and to top it all off, she did something I consider more horrible. One of my friends needed money so they could pay school bills for their kids. So their 16 year old daughter decided to help. I recommended her at work and they were going to hire her. Too bad that girl saw her. As soon as the interview was over, she followed the hiring manager into the office, and claimed that I was not "mentally stable" enough to recommend a hire. Furthermore, she claimed this girl was ugly, and needed to be more "pretty" to work fast food. And according to 5 different sources, she has bragged that she made the hiring call. She also slanders our head manager. I really want to report her to the manager, but how do I get proof?