I think you've gotten some good suggestions here already. One is to let her know that you fell she is MORE beautiful than her short-haired friends. Another is to let her know how beautiful she is in YOUR eyes. If there are others in your family, like your father, who feel the same, enlist him or them to support your effort to help her keep her long beautiful hair. If your family has a strong ethnic pride, like Indian or Indonesian, you can use that--it's more "proud of being Indian" to have long hair, and less "western." Showing her new and interesting ways to wear long hair is another good idea. If you want, go to my group--we have two folders dedicated to updo's right now.

I'd like to add one final suggestion. Offer her personal attention that makes it both easier to do the work involved with having long hair, and is rewarding for her. If you offer to help her comb it out, or dry it, or oil it, or even wash it once in a while, or read to her while she does it herself--whatever is comfortable and socially appropriate inside your family--that might be a reason for her to keep it, so she continues to enjoy the help and attention.

There are only 2 way to influence someone to do what you want. Convince them there's a payoff for them, or convince them to do it for you.

Good luck, pal. It's a mission worth the effort.

RJI