So, I searched Pinterest and found the original pin, which linked to a free pattern! I did a little happy dance, decided to make this my weekend project, and went to the yarn shop.
The only personalized request my sister had was that her headband be made in a bold lavender or lilac-colored yarn. I decided to use Bulky Weight Lacery yarn in Lilac Spray (note: the yarn is a bit darker and more complex in real life than it is in the pictures). The result was super gorgeous, super thick, and super soft. Her birthday is coming up in less than a week, and this is going to be the perfect gift.