“Arts institutions have been cultivating people in their 20s and 30s for years as a way of shoring up future donors. But Sarah Arison and Jenny Coyne are not merely passing through, writing a check and dressing up for a night in order to rub the right shoulders. They are among a small and privileged group who hope, and are being groomed, to do much more: to take over the family business, so to speak — that business being arts patronage.”