Published: Feb 03, 2014 7:19 PM EST

LEE COUNTY, Fla.- 9-year-old Makenzie Lawrey wrote a book called the Mighty Mito Superhero about her younger brother Gavin, to raise money and awareness for Mitochondrial Disease.

The disease causes seizures and leaves Gavin without much energy.

WINK News first introduced you to Gavin's family last year, and now his sister is doing her part to support Gavin.

Mom Brandi Lawrey says Makenzie has always written down her thoughts and feelings, and then she decided she wanted to turn them into a book.

"I wanted to write this book so there was a better way for people to understand what Mito is," the big sister said. She got in touch with Gavin's doctor and did her own research to write the book in a way everyone could understand. She also wrote about her own experiences helping to take care of Gavin.

"It makes me feel really good because I know that even if he doesn't feel good I can still help in a way," she says.

Makenzie told her mom she wanted to raise a million dollars to help find a cure for Mitochondrial Disease. While even Brandi thought it was a lofty goal, she thinks her daughter will achieve it.

In less than a week, Makenzie sold more than 500 books and collected about $20,000.

When paying for the book, Brandi encourages people to donate an amount equal to their age, to represent the life they hope Gavin can live.

For more information or to order the book, visit http://hope4mito.com.