For some reason Willow, the youngest sister, was the most difficult for me to write. In the first two books, Willow isn't in a lot of scenes, so I hadn't fleshed her out as much as the other three. This made "finding" her personality more tricky.
Also, in the first two books, she was sort of the "annoying younger sister" foil. That had been her purpose, so it was harder giving her some substance.
Willow and Ivy also share similarities in the same way as Linden and Maggie. Willow and Ivy are the popular, socially extroverted sisters. But where Ivy tends to be more independent and sometimes selfish, Willow is extremely close to and dependent on Linden. I enjoyed writing Linden's and Willow's relationship because they were so close. Originally, when I was writing the first book, I toyed with the idea of making them twins. Ultimately, I decided I wanted them to have a little more separation.
If you want to see Willow's board inspiration you can find it here: