My DD (15) is 5'9. She used to be a bit self-conscious but has got A LOT better recently. She told me she's decided to embrace being tall and actually bought a pair of wedges the other day!
The things that have helped (I think) are:
She plays netball - GD - where height is a great advantage. Could your DN be encouraged to get into netball (or basketball, as someone else suggested). It's wonderful for girls to be sporty and be involved in a club/ team outside school as well as in.
I often rave on about a couple of my older DD's friends, aged 17 and 18 who are fantastic role models. Tall, beautiful, sporty (again, netball players), great fashion sense, hard-working. They looked a bit tall when they were younger but now they are glamorous, leggy, modelly stunners with all of the good stuff (academics/ sport etc) going on for them as well. They have their pick of the alpha males because by upper sixth, there are plenty of six foot tall, gorgeous, clever, sporty boys to choose from. Are there any older girls around who would make good role models for your DN? Could you flag them up?
My daughter has also hung onto a comment that my older daughter's boyfriend made. He said the other day (to my fifteen year old) that he wouldn't ever want to go out with anyone shorter than 5'7 because it would look silly. I doubt it's true but I could have kissed him, she looked so delighted. I always tell her that lovely tall boys (like him - 6'3 and drop dead gorgeous) like lovely tall girls!
On a wider note though - it's hard to be a teen. They all just want to fit in and be in the middle of the pack. I talk to my girls about how every teenager - boys as well as girls - has things that make them feel different or that they feel self-conscious about. There are short boys, desparate to grow; big busted girls who are horribly self-conscious about their boobs; girls who are late with puberty who stuff bras with tissues and dread making excuses when it's swimming; kids with terrible acne that just want to hide; kids with red hair who get teased and (sadly) so on and on and on.
Would you take her out for girly shopping days - look around the shopping mall to see other tall girls who look great and take inspiration from their style? Try things on and play around with clothes to help her find the right look/ shapes/ lengths for her?
You sound like a lovely Auntie. Ultimately, for all of us, it's all about accepting who we are, learning to love and accept ourselves and learning to make the best of what we've been given. Remembering to count our blessings that we are healthy and that there are people who are so much worse off than us.
Best wishes.