I think 'fine-ness' is a physical state of being.... In other words, a woman (or man) 'grows' into being considered 'fine'.. And like most things, over time, fine-ness fades.... It sags, limps, beomces untoned, etc....
But sexiness is usually classified as a state of mind or atrtitude.. Most people that are considered sexy, are just that way naturally.... A women can have bedroom eyes. Or she may slink when she walks. A man can have a devilish grin or 'flow' when he walks. Those types of things characterize sexiness... And that usually never fades...
An examples:
Pam Grier was fine.... Bern Nadette Stanis (Thelma) was sexy....
Debbie Allen may've been fine. But Phylissa Rashad is sexy....
Vivica A. Fox is fine.... But Sally Richardson is sexy...
Lisa Bonet was fine.... But Jasmin Guy was sexy....
Toni Braxton is sexy.....
Jada Pinkett-Smith is sexy....