Is there any law whatsoever regarding being a personal trainer without a certification?

How about calling some professional organizations, such as the American College of Sports Medicine and/or the American Council on Exercise re: whether it's legal to provide some personal training sessions without the proper certification.

I don't believe there is a law that states you need to be certified, but just know that certification is not just a formality. You can still get insurance as a non-certified trainer, but it is more expensive and you are at a much greater risk for legal litigation. If someone gets hurt under your supervision and they sue you, the client's attorneys are going to rip you a new one for not being certified as well.

of course its legal, as long as you arent doing it for profit, which would require a license from your local gov. (a business license, not a personal trainers license.) if you are just helping out your aunt you have nothing to worry about.