The Otherworldlies (book 4?)

Fern and Sam McAllister are one of the oddest twins there could be. In fact they are almost opposite. Fern, an oddball, has midnight black hair, and is considerably smaller then her brother. Sam, a relatively popular boy, has blonde hair and is a couple of inches taller then his twin. They are so different that they are often times referred to as the salt and pepper twins. Fern often times has stomach aches that can make her double over, and has a high sensitivity to the sun, which leads to her "breakfast sunglasses" which she wears every morning on the way to school. Fern has many things that set her aside from other people, like the fact that she claims she can speak to the family dog, Byron, who she says speaks back, and the fact that she has no friends, aside from Sam.
So when Fern's two favorite people, Blythe Conrad and Lee Phillips try take tormenting Fern to far by "helping her out" with a haircut, Fern makes a new friend. Lindsey Lin, the absolute most popular girl in the whole school. And it is with Lindsey's help that she discovers a life altering secret.
At first, Fern is completely skeptical at the idea, quickly realizes that Lindsey's explanation makes the most since. Which means big trouble for Fern. This puts her in a position that she has no idea of what she should do, much less of what she could do to help with anything. Especially with her new found abilities seemingly getting out of hand.