I chanced on them again the other day (along with bits and pieces of a life I left behind, outdistanced long ago like the Isthmus that I once called home -) a few brief letters headed ''Dearest Sister -'' Like Aphrodite risen from the foam, like Athene, sprung armored from the brow of Zeus, she came to us from Johannesburg, full-grown: the daughter of my father's early marriage and gave unstintingly while she was there the headiest of gifts an adult can ever give a child - her time. It was endless then: we had the park, the beach, the pool, the garden's limes and mangoes, the palms crowned with orchids where chattering wild parakeets flew at dusk. And then there were the moments wholly ours like brushing my hair, adorning it with bows, sketching clever caricatures and reading stories at bedtime or on afternoons with windows shuttered from the tropic rain - unknowing that her face would ever afterward be superimposed on that of the Snow Queen and Ice Maiden - just as Marta, a sitter who came later, became fused with St. Lucy and St. Joan. I could not see beyond us to a time when she would not be there - but it came. There was for a little space, a trickle of letters with blurry postmarks and another name and no return address but the postal window, letters asking almost hungrily were the orange trees, the almond trees in bloom, was the sound of the sea still heard from her old room, asking about me: how much had I grown, asking about school and play and playmates, asking, always asking, never telling - then all too soon, only unbroken silence. There are things that are unrelated, strange or out of context, things that memory, like Chagall's ''Time Is a River Without Banks,'' somehow manages to juxtapose. I look at the yellowed letters in my hand and think, even if we never quite resolve the philosopher's Law of Three: Affirm, Deny and Reconcile - a scent, a sound, a scene restores it all. And sister, wherever you are can never be too far. There are things that even the clock and calendar have no power to blur.