Children do vary in the speed they develop skills. Some may be behind for awhile and then catch up. Also, you must consider your niece's ability level. Even if she is truly behind for a child with her aptitude, there are only a very few things that you can do to help her.
States have very different laws on homeschooling. Some don't even require parents to notify the state that they are homeschooling while others have high regulation requiring parents to send achievement test scores to the state, have the curriculum approved by the state, and even have home visits by state officials. You can easily find out what the legal requirements for your niece's state are by visiting the Regional and Worldwide Homeschooling site. In a state with high regulation, your niece might be required to return to public school if she is not meeting certain levels on achievement tests.
If you truly want to help your niece, your best option may be to actually take on some of the responsibility of her homeschooling. You could volunteer to tutor your niece for two hours, two days a week. Not only would your brother appreciate your help, you might also have the special skills needed to really make a difference with your niece in just this short period of tutoring. In each half hour of homeschooling, you would be able to accomplish as much as would be done in a typical classroom in two hours.