A sinker does not NECESSARILY have to come off the index finger. It varies from pitcher to pitcher --- and as you mentioned it could have something to do with the length of the fingers.
Some coaches teach pronation, I teach pressure points; both can create the same result. Even with pressure points, there's variation in method. For example, I tend to teach that a pitcher use his middle finger for all pressure (though it depends on the person). Tom Seaver believed in alternating pressure with the index or middle finger depending on the pitch.
In the end, a pitcher should be taught all the different methods until something "clicks", and/or he finds a comfort zone (and results!).
Just wanted to point out that there aren't necessarily any absolutes with this pitch --- generally, a pitcher can work on different grips and concepts and eventually find what works best for him.
Good luck and happy new year!