Just be up front about it. Apologize for not asking for your parents permission and explain why you didn't tell them in the first place. It's just a piercing. . . not a tattoo. If you think they are going to go crazy then maybe write them a note before you go to school to give them time to cool down about it when you get home.
Don't lie, own up to it and do it before your sister rats you out. After that, have a serious talk with your sister about how this is the kind of thing that you have to tell your parents about, and that it's not her business. That conversation will likely go nowhere, but you have to try it :P
You might want to add that piercings aren't permanent like the tattos your friends are getting. It's very difficult for anyone to argue that.
Of course, some piercings are more difficult to explain than others are. Explaining a belly button piercing as "I thought it looked cute!" and nipple piercings as...well, see that would be more difficult :P.