Honestly? Lead by example.
A young person subconsciously looks at those closest to them for cues on what "normal" is.
If they are surrounded by people who are cheerful and in healthy relationships (romantic or not), they will look for the same traits in others to bring into their life.
Live your life as good as you can, don't associate with trashy people (male or female), show her that you value the women you date for more than just their looks: How smart they are, how talented they are, kindness, etc... Drop little things every now and then like, "Man, I really like how smart *blank* is at XYZ." Or, "It's awesome that *blank* took time to travel to ABC and learned about their culture." and "*Blank* has got her life goals in check." Stuff other than how they look.
Use every opportunity you have to build up her self confidence. When she gets an A in a class, say something like "Man, my little sis is going to kick ass in college!". If she brings home an art project say something good about it! Wants to try a sport? Show up and cheer. Show her that she is a person, and should be appreciated for more than looking nice.
Do all that, and without thinking she will filter out all the sleazy people trying to get into her life.
Thanks, god bless you.
