My older sister had a party last week?

Hey my name is Sarah am 12 and last saturday my 16 year old sister Veronica http://img222.imageshack.us/my.php?image=70442431rt5.jpg had a party with her friends while my parents where away for the night. At around 8 she told to get ready for bed I said it's not my bed time yet but she told me to it's time for good little sisters to go to bed. I said ok but I was getting ready to go to sleep when I heard her friends come in and go down to are basement and I was listening on the floor of my room I could hear some music before I fell asleep. What were they doing down there? I think my older sister is the coolest person I know http://img18.imageshack.us/my.php?image=21517883dd2.jpg and she told me if I hear a nose not tell mom so now I don't know what to do?