Speeding Ticket/ Possession of Another liscense?

I am a 19 year old full time student. I just recently got pulled over for speeding 47 mph on a 30mph road. It was my first time speeding & he took a point off. At the same time when I was bringing out my drivers license, my sisters I.D. was at the opening. Usually my sister and I hang out a lot and it so happened that she left it in my wallet. As I was reaching in to give him my I.D. he saw that I had my sisters as well. He immediately took that from me & asked me why I had my sisters I.D. I told him that we usually switch purses and she must have left it there. He called my sister after and she said that we hung out the other day and she left it there. The cop immediately said that we both were lying and that we had two totally different stories. I have never used the I.D. in my entire life and he accused that I was using the I.D. to buy alcohol. Do also keep in mind that my sister and I work at the same place & she was working the same day as I. I was very nervous and scared when he pulled me over & I could have told him that I was bringing my sisters I.D. My sister is very forgetful and looses things and it so happened that the I.D. I had was the one she lost before. Please let me know what you think I should do and should I go to court for this case? Should I plead guilty? should I plead innocent? HELP PLEASE