I know they are related. They are both keratin.
If I can find my Milady's Nail Technology book, I'll get back to you with more info...
But yes, nine times out of ten, if what you put in your body is beneficial to your hair, it will also be beneficial to your nails and vice versa.
But if your body is depleted of nutrients or becoming ill, your nails are one of the first place to show signs.
Have you ever noticed that your doctor will grab your hands and flip them over to look at them? He's looking for signs of illness!
But in terms of topical treatments, such as shampoo and conditioner, etc., that wouldn't have a direct effect on nails, per se. Because that's external.
And if you do most of your hair care at home, you can also practice good nail care at home, and you will see your nails thrive just like your hair.
BTW, the oils used for hot oil treatments on the hair ( jojoba, avacado, evoo), are also great for giving yourself a hot oil manicure!
So when you're sitting under the dryer doing your deep conditioning, just put a little warm oil in a glass bowl and soak!
Blessings!