If you don't feel you would enjoy, really enjoy, restoring it either make sure it's in good dry storage and mothball it, or let it go. If the car is important to you keep it, as giving it to a relative means that you would witness any abuses done to it after your careful restoration and if the relative should crash it you'd probably bear some of the "blame". If anything, let the sweat equity and investment come from the sister's kid, then there should be more respect for the car. Maybe.

It sounds like other cars have your deeper interest. If you have the luxury of indulging all your interests, more power to you. If you are keeping it only because it was your high school car, how important is that to you? If not for that, would you still keep the car? Sentimentality can be expensive. If you have some particular memories that only that car can fill, then do right by it and make sure it's properly stored until you decide what you want to do.