Five years on, my sister contacted Michael Pollak of the New York Times, who answered her with the following:

Michael Miscione, the Manhattan borough historian, tracked it down.
The poem is "Tubby Hook," and it is included in a 1920 book, "Ballads of
Old New York" by Arthur Guiterman (1871-1943).

The narrative poem deals not with Spuyten Duyvil (the poem that follows
it does), but with Tubby Hook, known in Dutch as Tobbe Hoeck or "the
Cape of the Tub," a rocky point in Inwood about two-thirds of a mile
below Spuyten Duyvil.

As the legend goes, a Dutch housewife let nothing stand between her and
her washing, not even Easter Sunday. "Her earthly hope/Was placed in
soap." When her husband protested that work was forbidden on Easter, she
carried her tub down to the river and vowed, "I'll wash and wash till
the Judgment Day."

"Along a river that leaped in flame/the sailing witches of Salem came."
Each witch sat in a washtub, and each had a coal-black cat. "With cries
to Satan and Beelzebub/They shaped the cape like an upturned tub!" And
beneath it, the washerwoman is still there, trying to rub the fur of a
black cat white.

The delightfully illustrated book was reprinted in 2005, and both
original and reproduction are available on commercial Web sites.