Alcohol & Drugs: MIP, MIC, Intoxication minor possession or consumption of alcohol

[dotrr]

I got a complaint or a summons to court for minor consumption and possession of alcohol on private property without permission of a parent. I am 19 years old, my 22 year old sister was having her birthday party and the cops came because of a noise complaint. One cop came to my house, i am almost positive he was an undercover cop because he did not come in a police car. He asked for my info and i gave it to him and told him it was my sisters party. I said some people were under 21 but he did not id anyone or take further action. He just told everyone to "put down their drinks and go screw" i guess that meant leave. Some people left at this point and so did the cop. Today i got a complaint summons in the mail to appear in court.. i dont know if i should plead guilty because i was there or innocent because he has no proof i was drinking since he didnt ask or breathalize me.. and for the possession.. it was my sisters keg.. she is 21... what should i do and what are the consequences?? please help.. thanks