Go back to edit profile. Scroll down and you should be able to add or edit siblings. Click on that add/edit link.
Scroll down near the bottom of the page and it will tell you "To fix an incorrect sibling relationship you need to remove the incorrect parent" - I think they mean just from the profile. So for James Jr, you'd need to remove his brother as the father - that won't delete the profile. It lists all the names of siblings, so you might need to go into James Sr's profile - not sure there :)
Maybe someone else can help confirm this.I've never had to do this before, but understand it's an easy error to make!