My sister and I work in the same place. I handle the same money her and other employees handle. Is this a rightful termination?

I was contacted by the Human Resource Manager on regards to a job openning handling money. I agreed to come in for an interview and after fingerprinting and drug tests I was hired. Since I had worked in the company before and had only been away for about 3 months, I was never separated from the company. Therefore, I ask if I needed to fill out an application and they said no. When I first applied to the company years before, my sister and two other relatives were not working there. I did not think anything of it since I had been contacted by the HR Manger herself to let me know about the openning.