The answer is don't. If you're a nudist at 14 then that's your choice. Everyone makes their own choices in life whether it is right or wrong. You do what feels right in your heart and not what other people think you should do. I don't see the point in your sister having to be nude to help in making it a nude beach. You don't have to be nude to go to a nude beach. Be safe...
Erica, I can not agree with the Ice1q2w. Choices are generally made through education. If you don't know, it's difficult to make a choice. Therefore, 1st, I'd have to ask if you sister knows your a nudist? If so I see no problem in inviting her to a nude beach with you keeping safety in mind. If not, then I might suggest you start on a lighter note and introduce her to your world starting at home. There are many folks that wish they would have had the courage to go nude in public at a much earlier age than they did/are. I was young as well when I started in nudism/naturism. 40 years later I still enjoy it! Best thing that ever happened to me. Had it not been for my brother, I don't know when I would have discovered it.
And the other answers you received don't warrant an answer since I can see they are opposed to nudity in general and have already developed their own opinions... Probably without trying it.
I think you need to be arrested for even think that your a sick person who needs help joke or no joke if I knew you I would beat the crap out of you and then take your beat up body to the nude beach and put a sign on you reading piss on me I like little girls your sick and god will deal with that.