If you created your brother's profile then you can change his name. When you create a person on WikiTree you become the 'Profile Manager' which means you can change much of the person's information.
On your brother's profile near the top of the screen is a row of GREEN boxes. Click on EDIT and you will then be able to change his name.
You should also enter a reason why you are changing things in the box "Explain your changes".
Don't forget to scroll down the page and click on SAVE CHANGES after you change things!