Whenever a woman is photographed, the issue of her vanity is inevitably raised, but Nixon has finessed this with his choice of natural light, casual manner and unfussy preparation. The sisters never discuss what they are going to wear. Bebe Nixon says simply: “We just wear what we feel like wearing that day.”

Throughout this series, we watch these women age, undergoing life’s most humbling experience. While many of us can, when pressed, name things we are grateful to Time for bestowing upon us, the lines bracketing our mouths and the loosening of our skin are not among them. So while a part of the spirit sinks at the slow appearance of these women’s jowls, another part is lifted: They are not undone by it. We detect more sorrow, perhaps, in the eyes, more weight in the once-fresh brows. But the more we study the images, the more we see that aging does not define these women. Even as the images tell us, in no uncertain terms, that this is what it looks like to grow old, this is the irrefutable truth, we also learn: This is what endurance looks like.

It is the endurance of sisterhood in particular. Nixon, who grew up a single child, says he has always been particularly intrigued by the sisterly unit, and it shows in these images. With each passing year, the sisters seem to present more of a united front. Earlier assertions of their individuality — the arms folded across the chest, the standing apart — give way to a literal leaning on one another, as if independence is no longer such a concern. We see what goes on between the sisters in their bodies, particularly their limbs. A hand clasps a sister’s waist, arms embrace arms or are slung in casual solidarity over a shoulder. A palm steadies another’s neck, reassuring. The cumulative effect is dizzying and powerful. When 36 prints were exhibited in a gallery in Granada, Spain, viewers openly wept.